AArch64: Reserve two 64k pages for GIC CPU interface

On AArch64 system with a GICv2, the GICC range can be aligned
to the last 4k block of a 64k page, ending up straddling two
64k pages. In order not to conflict with the distributor mapping,
allocate two 64k pages to the CPU interface.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>