ARM: meson: rework soc arch file to prepare for new SoC

We are about to add support for the Amlogic AXG SoC. While very close to
the Gx SoC family, we will need to handle a few thing which are different
in this SoC. Rework the meson arch directory to prepare for this.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>