arm64: dts: meson-g12b-ugoos-am6: add initial device-tree

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

- 2GB LPDDR4 RAM
- 16GB eMMC storage
- 10/100/1000 Base-T Ethernet using External RGMII PHY
- 802.11 a/b/g/b/ac + BT 5.0 sdio wireless (Ampak 6398S)
- HDMI 2.0 (4k@60p) video
- Composite video + 2-channel audio output on 3.5mm jack
- S/PDIF audio output
- Aux input
- 1x USB 3.0
- 3x USB 2.0
- 1x micro SD card slot

The device-tree is largely based on meson-g12b-odroid-n2 but with audio
and USB config copied from meson-g12a-x96-max.

Tested-by: Oleg Ivanov <balbes-150@yandex.ru>
Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>