KVM: arm64: vgic-v2: Consolidate userspace access for MMIO registers

Align the GICv2 MMIO accesses from userspace with the way the GICv3
code is now structured.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>