KVM: arm/arm64: merge GICv3 RD_base and SGI_base register frames

Currently we handle the redistributor registers in two separate MMIO
regions, one for the overall behaviour and SPIs and one for the
SGIs/PPIs. That latter forces the creation of _two_ KVM I/O bus
devices for each redistributor.
Since the spec mandates those two pages to be contigious, we could as
well merge them and save the churn with the second KVM I/O bus device.

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