arm64: dts: meson-g12a: add initial g12a s905d2 SoC DT support

Try to add basic DT support for the Amlogic's Meson-G12A S905D2 SoC,
which describe components as follows: Reserve Memory, CPU, GIC, IRQ,
Timer, UART. It's capable of booting up into the serial console.

Signed-off-by: Jianxin Pan <jianxin.pan@amlogic.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>