sunxi: Add support for Allwinner A64 SoCs

The Allwinner A64 SoC is used in the Pine64. This patch adds
all bits necessary to compile U-Boot for it running in AArch64
mode.

Unfortunately SPL is not ready yet due to legal problems, so
we need to boot using the binary boot0 for now.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
[agraf: remove SPL code, move to AArch64]
Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>