ARM: meson: Add support for AXG family

ARM: meson: Add support for AXG family

This patch adds support for the Amlogic AXG SoC, which is very close from
the Amlogic GXL SoCs with :
- Same 4xCortex-A53 CPUs but clocked at 1.2GHZ max
- DDR Interface limited to DDR4 16bit
- The whole physical register address space has been moved to 0xfxxxxxxx
- The pinctrl setup has changed
- The clock tree is different enough to use a different driver

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