arm64: dts: meson-axg: add initial A113D SoC DT support

Try to add basic DT support for the Amlogic's Meson-AXG A113D 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: Yixun Lan <yixun.lan@amlogic.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>