arm64: cpufeature: Expose get_arm64_ftr_reg() outside cpufeature.c

arm64: cpufeature: Expose get_arm64_ftr_reg() outside cpufeature.c

get_arm64_ftr_reg() returns the properties of a system register based
on its instruction encoding.

This is needed by erratum workaround in cpu_errata.c to modify the
user-space visible view of id registers.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220909165938.3931307-3-james.morse@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>