ARM64: dts: meson-gxbb-odroidc2: Enable SARADC node

Now that 3adbf342

 "iio: adc: add a driver for the SAR ADC found in
Amlogic Meson SoCs" has added support for the ADC, let's enable it
on Odroid C2.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>