sunxi: add board documentation

Add some long overdue instructions for building and installing U-Boot on
Allwinner SoC based boards.
This describes the building process, including TF-A and crust, plus
installation to SD card, eMMC and SPI flash, both from Linux and U-Boot
itself. Also describe FEL booting.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>