ARM: dts: meson: add the SAR ADC

This adds the SAR ADC to meson.dtsi and configures the clocks on Meson8
and Meson8b to allow boards to use it. Some boards use it to connect a
button to it.

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