Documentation: dt-bindings: add the Amlogic Meson SAR ADC documentation

This adds the devicetree binding documentation for the SAR ADC found in
Amlogic Meson SoCs.
Currently only the GXBB, GXL and GXM SoCs are supported.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/amlogic,meson-saradc.txt

+* Amlogic Meson SAR (Successive Approximation Register) A/D converter
+
+Required properties:
+- compatible:	depending on the SoC this should be one of:
+			- "amlogic,meson-gxbb-saradc" for GXBB
+			- "amlogic,meson-gxl-saradc" for GXL
+			- "amlogic,meson-gxm-saradc" for GXM
+		along with the generic "amlogic,meson-saradc"
+- reg:		the physical base address and length of the registers
+- clocks:	phandle and clock identifier (see clock-names)
+- clock-names:	mandatory clocks:
+			- "clkin" for the reference clock (typically XTAL)
+			- "core" for the SAR ADC core clock
+		optional clocks:
+			- "sana" for the analog clock
+			- "adc_clk" for the ADC (sampling) clock
+			- "adc_sel" for the ADC (sampling) clock mux
+- vref-supply:	the regulator supply for the ADC reference voltage
+- #io-channel-cells: must be 1, see ../iio-bindings.txt
+
+Example:
+	saradc: adc@8680 {
+		compatible = "amlogic,meson-gxl-saradc", "amlogic,meson-saradc";
+		#io-channel-cells = <1>;
+		reg = <0x0 0x8680 0x0 0x34>;
+		clocks = <&xtal>,
+			 <&clkc CLKID_SAR_ADC>,
+			 <&clkc CLKID_SANA>,
+			 <&clkc CLKID_SAR_ADC_CLK>,
+			 <&clkc CLKID_SAR_ADC_SEL>;
+		clock-names = "clkin", "core", "sana", "adc_clk", "adc_sel";
+	};