arm64: atomics: implement atomic{,64}_cmpxchg using cmpxchg

We don't need duplicate cmpxchg implementations, so use cmpxchg to
implement atomic{,64}_cmpxchg, like we do for xchg already.

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>

diff --cc arch/arm64/include/asm/atomic.h
Simple merge

diff --cc arch/arm64/include/asm/atomic_ll_sc.h
Simple merge

diff --cc arch/arm64/include/asm/atomic_lse.h
Simple merge