ARM64: dts: meson-gxl: add support for the Oranth Tanix TX3 Mini

The Tanix TX3 Mini is a TV box based on the Amlogic S905W chipset.
There are two variants:
- 1 GiB or 2 GiB of DDR3 memory
- 8 GB or 16 GB eMMC flash

Both variants come with:
- 802.11 b/g/n wifi (Silicon Valley Microelectronics SSV6051, does not
  support Bluetooth)
- an LED 7 segment display with an FD628 controller
- HDMI and AV (CVBS) output
- 2x USB (utilizing both USB ports provided by the SoC)
- micro SD card slot
- serial console (uart_AO) has to be soldered after opening the case

The board seems to be very similar to the P23x and Q20x reference
boards, which is why it includes meson-gx-p23x-q20x.dtsi:
- eMMC reset routed to BOOT_9
- the SDIO wifi chip's reset line is routed to GPIOX_6 and the reference
  clock is 32.768KHz on PWM_E
- SD card detection is routed to CARD_6
- vqmmc of all MMC controllers is hard-wired to 1.8V (VDDIO_BOOT)
- uart_AO can be accessed after opening the case and soldering RX, TX
  and GND lines onto the exposed solder points (marked with RX, TX and
  GND)

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