soc: amlogic: meson-gx-pwrc-vpu: Add support for G12A

The Amlogic G12A SoC has a very similar VPU Power Controller setup
than the older GXBB, GXL & GXm SoCs.

This patch adds the variant support for G12A.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>