allwinner: Introduce basic platform support

This platform supports Allwinner's SoCs with ARMv8 cores. So far they
all sport a single cluster of Cortex-A53 cores.

"sunxi" is the original code name used for this platform, and since it
appears in the Linux kernel and in U-Boot as well, we use it here as a
short file name prefix and for identifiers.

This port includes BL31 support only. U-Boot's SPL takes the role of the
primary loader, also doing the DRAM initialization. It then loads the
rest of the firmware, namely ATF and U-Boot (BL33), then hands execution
over to ATF.

This commit includes the basic platform code shared across all SoCs.
There is no platform.mk yet.

[Andre: moved files into proper directories, supported RESET_TO_BL31,
	various clean ups and simplifications ]

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