arm64/HWCAP: Use system wide safe values

Extend struct arm64_cpu_capabilities to handle the HWCAP detection
and make use of the system wide value of the feature registers for
a reliable set of HWCAPs.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Tested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>