dt-bindings: arm: amlogic: Add support for the Ugoos AM6

The Ugoos AM6 is based on the Amlogic W400 (G12B) reference design using the
S922X chipset.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>