arm64: dts: meson: g12: add a g12 layer

While the sm1 is very close to the g12a/b family, somethings apply
differently on the g12a/b and not the sm1. This introduce a new layer
of dtsi for part which apply to the g12a and g12b but not the sm1.

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