arm64: mops: detect and enable FEAT_MOPS

The Arm v8.8/9.3 FEAT_MOPS feature provides new instructions that
perform a memory copy or set. Wire up the cpufeature code to detect the
presence of FEAT_MOPS and enable it.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>