dt-bindings: arm: sunxi: Add Tanix TX6 mini

Tanix TX6 mini is, as its name suggest, very similar to Tanix TX6, but
with less features. It misses bluetooth support, it has less RAM, wifi
supports only 2.4G, it comes with different IR remote, etc.

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