arm64: dts: meson: convert ugoos-am6 to common w400 dtsi

Convert the Ugoos AM6 dts into meson-g12b-w400.dtsi, and then create
a new AM6 dts that references it. This is in preparation for several
new G12B devices that will share the common w400 dtsi.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200520014329.12469-2-christianshewitt@gmail.com