ARM64: dts: meson-gxl: add support for the Khadas VIM board

The Khadas VIM series consists of two boards which are almost
identical:
They are both using the same GXL S905X SoC, 100Mbit/s ethernet
(through the SoC-internal PHY), 2GB DDR3 memory, a micro-SD card slot,
onboard eMMC, Broadcom based SDIO WIFI, 2x USB A and 1x USB Type-C (the
latter with OTG support). The red LED is driven by PWM_AO_B (which
allows dimming), while the blue LED is managed by the firmware.
The differences are:
- the VIM Pro has a 16GB eMMC module, while the VIM only has 8GB
- the VIM Pro uses an AP6255 a/b/g/n/ac WIFI module, while the VIM comes
  with an AP6212 b/g/n SDIO WIFI module
 (the Vim uses an 8GB eMMC module, while

The boards are based on Amlogic's GXL S905X P212 reference design, which
is why most of the functionality (all MMC controllers and power
sequences, IR remote input, the main UART, ADC and ethernet) is simply
inherited from meson-gxl-s905x-p212.dtsi.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>