arm64: alternatives: add cpu feature for lse atomics

Add a CPU feature for the LSE atomic instructions, so that they can be
patched in at runtime when we detect that they are supported.

Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>