We don't need duplicate cmpxchg implementations, so use cmpxchg to
implement atomic{,64}_cmpxchg, like we do for xchg already.
Reviewed-by: NSteve Capper <steve.capper@arm.com>
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>