arm64: make ARM64_HAS_GIC_PRIO_MASKING depend on ARM64_HAS_GIC_CPUIF_SYSREGS

arm64: make ARM64_HAS_GIC_PRIO_MASKING depend on ARM64_HAS_GIC_CPUIF_SYSREGS

Currently the arm64_cpu_capabilities structure for
ARM64_HAS_GIC_PRIO_MASKING open-codes the same CPU field definitions as
the arm64_cpu_capabilities structure for ARM64_HAS_GIC_CPUIF_SYSREGS, so
that can_use_gic_priorities() can use has_useable_gicv3_cpuif().

This duplication isn't ideal for the legibility of the code, and sets a
bad example for any ARM64_HAS_GIC_* definitions added by subsequent
patches.

Instead, have ARM64_HAS_GIC_PRIO_MASKING check for the
ARM64_HAS_GIC_CPUIF_SYSREGS cpucap, and add a comment explaining why
this is safe. Subsequent patches will use the same pattern where one
cpucap depends upon another.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230130145429.903791-4-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>