sunxi: h616: add Tanix TX1 support

The Tanix TX1 is a tiny TV box, featuring the Allwinner H313 SoC with up
to 2GB of DRAM and 16GB of eMMC. There is no SD card or Ethernet port on
this small device, but it can be booted via the USB debug "FEL" mode.
The bootloader could then be written to the eMMC.

Add the defconfig for that board, and add the devicetree file to the
Makefile, for it to be built.
The DRAM parameters were taken from the vendor firmware on the eMMC.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>