iio: adc: Add Renesas GyroADC bindings

Add DT bindings for the Renesas RCar GyroADC block. This block is
a simple 4/8-channel ADC which samples 12/15/24 bits of data every
cycle from all channels.
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Simon Horman <horms+renesas@verge.net.au>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-renesas-soc@vger.kernel.org
Cc: Wolfram Sang <wsa@the-dreams.de>
Cc: Rob Herring <robh@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/renesas,gyroadc.txt

+* Renesas RCar GyroADC device driver
+
+The GyroADC block is a reduced SPI block with up to 8 chipselect lines,
+which supports the SPI protocol of a selected few SPI ADCs. The SPI ADCs
+are sampled by the GyroADC block in a round-robin fashion and the result
+presented in the GyroADC registers.
+
+Required properties:
+- compatible:	Should be "<soc-specific>", "renesas,rcar-gyroadc".
+                The <soc-specific> should be one of:
+		renesas,r8a7791-gyroadc - for the GyroADC block present
+					  in r8a7791 SoC
+		renesas,r8a7792-gyroadc - for the GyroADC with interrupt
+					  block present in r8a7792 SoC
+- reg:		Address and length of the register set for the device
+- clocks:	References to all the clocks specified in the clock-names
+		property as specified in
+		Documentation/devicetree/bindings/clock/clock-bindings.txt.
+- clock-names:	Shall contain "fck" and "if". The "fck" is the GyroADC block
+		clock, the "if" is the interface clock.
+- power-domains: Must contain a reference to the PM domain, if available.
+- #address-cells: Should be <1> (setting for the subnodes) for all ADCs
+		except for "fujitsu,mb88101a". Should be <0> (setting for
+		only subnode) for "fujitsu,mb88101a".
+- #size-cells:	Should be <0> (setting for the subnodes)
+
+Sub-nodes:
+You must define subnode(s) which select the connected ADC type and reference
+voltage for the GyroADC channels.
+
+Required properties for subnodes:
+- compatible:	Should be either of:
+		"fujitsu,mb88101a"
+			- Fujitsu MB88101A compatible mode,
+			  12bit sampling, up to 4 channels can be sampled in
+			  round-robin fashion. One Fujitsu chip supplies four
+			  GyroADC channels with data as it contains four ADCs
+			  on the chip and thus for 4-channel operation, single
+			  MB88101A is required. The Cx chipselect lines of the
+			  MB88101A connect directly to two CHS lines of the
+			  GyroADC, no demuxer is required. The data out line
+			  of each MB88101A connects to a shared input pin of
+			  the GyroADC.
+		"ti,adcs7476" or "ti,adc121" or "adi,ad7476"
+			- TI ADCS7476 / TI ADC121 / ADI AD7476 compatible mode,
+			  15bit sampling, up to 8 channels can be sampled in
+			  round-robin fashion. One TI/ADI chip supplies single
+			  ADC channel with data, thus for 8-channel operation,
+			  8 chips are required. A 3:8 chipselect demuxer is
+			  required to connect the nCS line of the TI/ADI chips
+			  to the GyroADC, while MISO line of each TI/ADI ADC
+			  connects to a shared input pin of the GyroADC.
+		"maxim,max1162" or "maxim,max11100"
+			- Maxim MAX1162 / Maxim MAX11100 compatible mode,
+			  16bit sampling, up to 8 channels can be sampled in
+			  round-robin fashion. One Maxim chip supplies single
+			  ADC channel with data, thus for 8-channel operation,
+			  8 chips are required. A 3:8 chipselect demuxer is
+			  required to connect the nCS line of the MAX chips
+			  to the GyroADC, while MISO line of each Maxim ADC
+			  connects to a shared input pin of the GyroADC.
+- reg:		Should be the number of the analog input. Should be present
+		for all ADCs except "fujitsu,mb88101a".
+- vref-supply:	Reference to the channel reference voltage regulator.
+
+Example:
+	vref_max1162: regulator-vref-max1162 {
+		compatible = "regulator-fixed";
+
+		regulator-name = "MAX1162 Vref";
+		regulator-min-microvolt = <4096000>;
+		regulator-max-microvolt = <4096000>;
+	};
+
+	adc@e6e54000 {
+		compatible = "renesas,r8a7791-gyroadc", "renesas,rcar-gyroadc";
+		reg = <0 0xe6e54000 0 64>;
+		clocks = <&mstp9_clks R8A7791_CLK_GYROADC>, <&clk_65m>;
+		clock-names = "fck", "if";
+		power-domains = <&sysc R8A7791_PD_ALWAYS_ON>;
+
+		pinctrl-0 = <&adc_pins>;
+		pinctrl-names = "default";
+
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		adc@0 {
+			reg = <0>;
+			compatible = "maxim,max1162";
+			vref-supply = <&vref_max1162>;
+		};
+
+		adc@1 {
+			reg = <1>;
+			compatible = "maxim,max1162";
+			vref-supply = <&vref_max1162>;
+		};
+	};