The GIC system registers are accessed using open-coded wrappers around
the mrs_s/msr_s asm macros.

This patch moves the code over to the {read,wrote}_sysreg_s accessors
instead, reducing the amount of explicit asm blocks in the arch headers.
Reviewed-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>