pinctrl: meson: rework gx pmx function

In preparation of supporting the new Amlogix AGX SoCs, we need to move
the Amlogic GX pinmux functions out of the common code to be able to
add a different set of SoC specific pinmux functions for AXG.

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