arm64: Use build-time assertions for cpucap ordering

Both sme2_kernel_enable() and fa64_kernel_enable() need to run after
sme_kernel_enable(). This happens to be true today as ARM64_SME has a
lower index than either ARM64_SME2 or ARM64_SME_FA64, and both functions
have a comment to this effect.

It would be nicer to have a build-time assertion like we for for
can_use_gic_priorities() and has_gic_prio_relaxed_sync(), as that way
it will be harder to miss any potential breakage.

This patch replaces the comments with build-time assertions.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/fpsimd.c

@@ -1310,23 +1310,21 @@ void sme_kernel_enable(const struct arm64_cpu_capabilities *__always_unused p)
 	isb();
 }
 
-/*
- * This must be called after sme_kernel_enable(), we rely on the
- * feature table being sorted to ensure this.
- */
 void sme2_kernel_enable(const struct arm64_cpu_capabilities *__always_unused p)
 {
+	/* This must be enabled after SME */
+	BUILD_BUG_ON(ARM64_SME2 <= ARM64_SME);
+
 	/* Allow use of ZT0 */
 	write_sysreg_s(read_sysreg_s(SYS_SMCR_EL1) | SMCR_ELx_EZT0_MASK,
 		       SYS_SMCR_EL1);
 }
 
-/*
- * This must be called after sme_kernel_enable(), we rely on the
- * feature table being sorted to ensure this.
- */
 void fa64_kernel_enable(const struct arm64_cpu_capabilities *__always_unused p)
 {
+	/* This must be enabled after SME */
+	BUILD_BUG_ON(ARM64_SME_FA64 <= ARM64_SME);
+
 	/* Allow use of FA64 */
 	write_sysreg_s(read_sysreg_s(SYS_SMCR_EL1) | SMCR_ELx_FA64_MASK,
 		       SYS_SMCR_EL1);