ARM: dts: meson8b: odroidc1: enable the SAR ADC

Odroid-C1 exposes ADC channels 0 and 1 on the GPIO headers. NOTE: Due
to the SoC design these are limited to 1.8V (instead of 3.3V like all
other pins).
Enable the SAR ADC to enable voltage measurements on these pins.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>