arm64: cpufeature: Detect CPU RAS Extentions

ARM's v8.2 Extentions add support for Reliability, Availability and
Serviceability (RAS). On CPUs with these extensions system software
can use additional barriers to isolate errors and determine if faults
are pending. Add cpufeature detection.

Platform level RAS support may require additional firmware support.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
[Rebased added config option, reworded commit message]
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>