MIPS: atomic: Deduplicate 32b & 64b read, set, xchg, cmpxchg

Remove the remaining duplication between 32b & 64b in asm/atomic.h by
making use of an ATOMIC_OPS() macro to generate:

  - atomic_read()/atomic64_read()
  - atomic_set()/atomic64_set()
  - atomic_cmpxchg()/atomic64_cmpxchg()
  - atomic_xchg()/atomic64_xchg()

This is consistent with the way all other functions in asm/atomic.h are
generated, and ensures consistency between the 32b & 64b functions.

Of note is that this results in the above now being static inline
functions rather than macros.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org