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

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

Commit 72c583951526 (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>
(cherry picked from commit f4c321eb268e932786c112e0b902ee942d91a336)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>