dt-bindings: arm: amlogic: Add support for the Oranth Tanix TX3 Mini

The Tanix TX3 Mini is a TV box based on the Amlogic S905W chipset. It
comes with:
- 1 GiB or 2 GiB of DDR3 memory depending on the model
- 8 GB or 16 GB eMMC flash depending on the model
- 802.11 b/g/n wifi (Silicon Valley Microelectronics SSV6051, does not
  support Bluetooth, not supported by any mailine driver)
- an LED 7 segment display with an FD628 controller (not supported by
  any mainline driver)
- HDMI and AV (CVBS) output
- 2x USB (utilizing both USB ports provided by the SoC)
- micro SD card slot

The board seems to be very similar to the P23x and Q20x reference
boards:
- 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: Rob Herring <robh@kernel.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>