dt-bindings: arm: amlogic: add Amlogic AD401 bindings

Add the compatible for the Amlogic A1 Based AD401 board.

Signed-off-by: Jianxin Pan <jianxin.pan@amlogic.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>