arm64: dts: meson: g12: factor the power domain.

The power domain declared in the g12a and g12b dtsi are the same.
Move the declaration of these power domains in the g12 common dtsi.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>