arm64: KVM: GICv3: move system register access to msr_s/mrs_s

Commit 72c58395 (arm64: gicv3: Allow GICv3 compilation with
older binutils) changed the way we express the GICv3 system registers,
but couldn't change the occurences used by KVM as the code wasn't
merged yet.

Just fix the accessors.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>