dt-bindings: power: amlogic, meson-gx-pwrc: Add G12A compatible

The Amlogic G12A has a slighly different Power Control, but uses the
same address space and sysctrl registers.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>