csky: Emulate one-byte cmpxchg

csky: Emulate one-byte cmpxchg

Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on csky.

[ paulmck: Apply kernel test robot feedback. ]
[ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]

Co-developed-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: <linux-csky@vger.kernel.org>