arm64: cpufeatures: Add missing ID_AA64MMFR4_EL1 to __read_sysreg_by_encoding()

When triggering a CPU hotplug scenario, we reparse the CPU feature
with SCOPE_LOCAL_CPU, for which we use __read_sysreg_by_encoding()
to get the HW value for this CPU.

As it turns out, we're missing the handling for ID_AA64MMFR4_EL1,
and trigger a BUG(). Funnily enough, Marek isn't completely happy
about that.

Add the damn register to the list.

Fixes: 805bb61f ("arm64: cpufeature: Add ID_AA64MMFR4_EL1 handling")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240212144736.1933112-2-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kernel/cpufeature.c

@@ -1456,6 +1456,7 @@ u64 __read_sysreg_by_encoding(u32 sys_id)
 	read_sysreg_case(SYS_ID_AA64MMFR1_EL1);
 	read_sysreg_case(SYS_ID_AA64MMFR2_EL1);
 	read_sysreg_case(SYS_ID_AA64MMFR3_EL1);
+	read_sysreg_case(SYS_ID_AA64MMFR4_EL1);
 	read_sysreg_case(SYS_ID_AA64ISAR0_EL1);
 	read_sysreg_case(SYS_ID_AA64ISAR1_EL1);
 	read_sysreg_case(SYS_ID_AA64ISAR2_EL1);