allwinner: Add Allwinner A64 support

The Allwinner A64 SoC is quite popular on single board computers.
It comes with four Cortex-A53 cores in a singe cluster and the usual
peripherals for set-top box/tablet SoC.

The ATF platform target is called "sun50i_a64".

[Andre: adapted to amended directory layout, removed unneeded definitions ]

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>