ARM: meson: add G12a support

Add support for the Amlogic G12A SoC, which is a mix between the
new physical memory mapping of AXG and the functionnalities of
the previous Amlogic GXL/GXM SoCs.

To handle the internal ethernet PHY, the Amlogic G12A SoCs now
embeds a dedicated PLL to feed the internal PHY.

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