arm64: dts: add support for A1 based Amlogic AD401

Add basic support for the Amlogic A1 based Amlogic AD401 board:
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>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>