Add a cpufeature indicating whether a cpu supports masking interrupts
by priority.

The feature will be properly enabled in a later patch.
Signed-off-by: NJulien Thierry <julien.thierry@arm.com>
Reviewed-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>