sunxi: boot0.h: allow RVBAR MMIO address customisation

To switch the ARMv8 Allwinner SoCs into the 64-bit AArch64 ISA, we need
to program the 64-bit start code address into an MMIO mapped register
that shadows the architectural RVBAR register.
This address is SoC specific, with just two versions out there so far.
Now a third address emerged, on a *variant* of an existing SoC (H616).

Change the boot0.h start code to make this address a Kconfig
selectable option, to allow easier maintenance.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>