dt-bindings: iio: adc: add binding for X-Powers AXP PMICs ADC

X-Powers PMICs have several ADC channels that can be used for different
purposes, e.g. PMIC internal temperature, battery voltage or AC current.

This is the documentation for AXP209, AXP221/223 and AXP813 ADC
bindings.
Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/axp20x_adc.txt

+* X-Powers AXP ADC bindings
+
+Required properties:
+  - compatible: should be one of:
+    - "x-powers,axp209-adc",
+    - "x-powers,axp221-adc",
+    - "x-powers,axp813-adc",
+  - #io-channel-cells: should be 1,
+
+Example:
+
+&axp22x {
+	adc {
+		compatible = "x-powers,axp221-adc";
+		#io-channel-cells = <1>;
+	};
+};
+
+ADC channels and their indexes per variant:
+
+AXP209
+------
+ 0 | acin_v
+ 1 | acin_i
+ 2 | vbus_v
+ 3 | vbus_i
+ 4 | pmic_temp
+ 5 | gpio0_v
+ 6 | gpio1_v
+ 7 | ipsout_v
+ 8 | batt_v
+ 9 | batt_chrg_i
+10 | batt_dischrg_i
+
+AXP22x
+------
+ 0 | pmic_temp
+ 1 | batt_v
+ 2 | batt_chrg_i
+ 3 | batt_dischrg_i
+
+AXP813
+------
+ 0 | pmic_temp
+ 1 | gpio0_v
+ 2 | batt_v
+ 3 | batt_chrg_i
+ 4 | batt_dischrg_i