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>