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

* 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";

	};