ARM64: dts: meson: meson-gx: add the SAR ADC

Add the SAR ADC to meson-gxbb.dtsi and meson-gxl.dtsi. GXBB provides a
10-bit ADC while GXL and GXM provide a 12-bit ADC.
Some boards use resistor ladder buttons connected through one of the ADC
channels. On newer devices (GXL and GXM) some boards use pull-ups/downs
to change the resistance (and thus the ADC value) on one of the ADC
channels to indicate the board revision.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>