Documentation: devicetree: add epson rx6110 binding

Add the binding documentation for the Epson RX6110 RTC.
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/epson,rx6110.txt

+Epson RX6110 Real Time Clock
+============================
+
+The Epson RX6110 can be used with SPI or I2C busses. The kind of
+bus depends on the SPISEL pin and can not be configured via software.
+
+I2C mode
+--------
+
+Required properties:
+  - compatible: should be: "epson,rx6110"
+  - reg : the I2C address of the device for I2C
+
+Example:
+
+	rtc: rtc@32 {
+		compatible = "epson,rx6110"
+		reg = <0x32>;
+	};
+
+SPI mode
+--------
+
+Required properties:
+  - compatible: should be: "epson,rx6110"
+  - reg: chip select number
+  - spi-cs-high: RX6110 needs chipselect high
+  - spi-cpha: RX6110 works with SPI shifted clock phase
+  - spi-cpol: RX6110 works with SPI inverse clock polarity
+
+Example:
+
+	rtc: rtc@3 {
+		compatible = "epson,rx6110"
+		reg = <3>
+		spi-cs-high;
+		spi-cpha;
+		spi-cpol;
+	};