dt-bindings: iio: adc: add Meson8 and Meson8b support

The Amlogic Meson SAR ADC driver can be used on Meson8 and Meson8b
(probably on earlier SoC generations as well, but I don't have any
hardware available for testing that).
Add a separate compatible for Meson8 and Meson8b because it does not
need any of the BL30 magic (unlike the GX SoCs).
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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

@@ -2,6 +2,8 @@
 
 Required properties:
 - compatible:	depending on the SoC this should be one of:
+			- "amlogic,meson8-saradc" for Meson8
+			- "amlogic,meson8b-saradc" for Meson8b
 			- "amlogic,meson-gxbb-saradc" for GXBB
 			- "amlogic,meson-gxl-saradc" for GXL
 			- "amlogic,meson-gxm-saradc" for GXM