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.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/cpufeature.c

@@ -2046,7 +2046,15 @@ early_param("irqchip.gicv3_pseudo_nmi", early_enable_pseudo_nmi);
 static bool can_use_gic_priorities(const struct arm64_cpu_capabilities *entry,
 				   int scope)
 {
-	return enable_pseudo_nmi && has_useable_gicv3_cpuif(entry, scope);
+	/*
+	 * ARM64_HAS_GIC_CPUIF_SYSREGS has a lower index, and is a boot CPU
+	 * feature, so will be detected earlier.
+	 */
+	BUILD_BUG_ON(ARM64_HAS_GIC_PRIO_MASKING <= ARM64_HAS_GIC_CPUIF_SYSREGS);
+	if (!cpus_have_cap(ARM64_HAS_GIC_CPUIF_SYSREGS))
+		return false;
+
+	return enable_pseudo_nmi;
 }
 #endif
 
@@ -2537,11 +2545,6 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.capability = ARM64_HAS_GIC_PRIO_MASKING,
 		.type = ARM64_CPUCAP_STRICT_BOOT_CPU_FEATURE,
 		.matches = can_use_gic_priorities,
-		.sys_reg = SYS_ID_AA64PFR0_EL1,
-		.field_pos = ID_AA64PFR0_EL1_GIC_SHIFT,
-		.field_width = 4,
-		.sign = FTR_UNSIGNED,
-		.min_field_value = 1,
 	},
 #endif
 #ifdef CONFIG_ARM64_E0PD