The header files in arm/aarch*/include/asm/ are directly copied from
Linux, so we can't just put our own definitions in there.
Move the GICv2M MMIO frame size into a more private header, to avoid
breaking the build once the header files are synced from Linux.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>