arm64: dts: allwinner: h6: Add Tanix TX6 mini dts

Tanix TX6 mini is less capable version of Tanix TX6 although it comes
with some features not present on Tanix TX6.

Basic specs:
- H6 SoC
- 2 GiB DDR3 RAM
- HDMI
- SPDIF
- 2x USB
- analogue audio
- CVBS
- SD card
- IR remote
- LED display
- fast ethernet
- XR819 wifi
- 16 GiB eMMC

Currently supported features doesn't differ that much from Tanix TX6,
but that will change soon.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>