arm64: dts: meson-g12b-khadas-vim3: add initial device-tree

The Khadas VIM3 uses the Amlogic S922X or A311S SoC, both based on the
Amlogic G12B SoC family, on a board with the same form factor as the
VIM/VIM2 models. It ships in two variants; basic and
pro which differ in RAM and eMMC size:

- 2GB (basic) or 4GB (pro) LPDDR4 RAM
- 16GB (basic) or 32GB (pro) eMMC 5.1 storage
- 16MB SPI flash
- 10/100/1000 Base-T Ethernet
- AP6398S Wireless (802.11 a/b/g/n/ac, BT5.0)
- HDMI 2.1 video
- 1x USB 2.0 + 1x USB 3.0 ports
- 1x USB-C (power) with USB 2.0 OTG
- 3x LED's (1x red, 1x blue, 1x white)
- 3x buttons (power, function, reset)
- IR receiver
- M2 socket with PCIe, USB, ADC & I2C
- 40pin GPIO Header
- 1x micro SD card slot

A common meson-g12b-khadas-vim3.dtsi is added to support both S922X and
A311D SoCs supported by two variants of the board.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>