This patch moves the GICv3 system register access helpers to
arch/arm64/. Their 32bit counterparts will need to use mrc/mcr accesses
instead of mrs_s/msr_s.
[maz: fixed conflict with Cavium erratum handling] Reviewed-by: Marc Zyngier <marc.zyngier@arm.com> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>