sunxi: Add support for Tanix TX6

This commit adds support for Tanix TX6 TV box, based on H6. It's low end
H6 board, with 3 GiB of RAM, eMMC, fast ethernet, USB, IR and other
peripherals.

DT file is taken from Linux 5.11-rc1 release.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>