dt-bindings: iio: adc: document the Meson AXG support

Update the documentation to expicitly support the Meson-AXG SoC.
Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -7,6 +7,7 @@ Required properties:
 			- "amlogic,meson-gxbb-saradc" for GXBB
 			- "amlogic,meson-gxl-saradc" for GXL
 			- "amlogic,meson-gxm-saradc" for GXM
+			- "amlogic,meson-axg-saradc" for AXG
 		along with the generic "amlogic,meson-saradc"
 - reg:		the physical base address and length of the registers
 - interrupts:	the interrupt indicating end of sampling