arm64: dts: allwinner: h6: tanix-tx6: Split to DT and DTSI

There is another very similar device to Tanix TX6, namely Tanix TX6
mini. Because most of the board design is shared, it makes sense to have
common nodes in DTSI file.

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