sunxi: A64: enable SPL

Now that the SPL is ready to be compiled in AArch64 and the DRAM
init code is ready, enable SPL support for the A64 SoC and in the
Pine64 defconfig.
For now we keep the boot0 header in the U-Boot proper, as this allows
to still use boot0 as an SPL replacement without hurting the SPL use
case.
We disable FEL support for now by making its compilation conditional
and disabling it for ARM64, as the code isn't ready yet.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>