arm/arm64: gic-v3: Add PMR and RPR accessors

Add helper functions to access system registers related to interrupt
priorities: PMR and RPR.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>