Open-coding the pointer-auth HWCAPs is a mess and can be avoided by
reusing the multi-cap logic from the CPU errata framework.

Move the multi_entry_cap_matches code to cpufeature.h and reuse it for
the pointer auth HWCAPs.
Reviewed-by: NSuzuki Poulose <suzuki.poulose@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>