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 <[e-mail]>
Signed-off-by: Yujie Liu <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Tested-by: Yujie Liu <[e-mail]>
Reviewed-by: Guo Ren <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: <[e-mail]>

arch/csky/Kconfig

···
 	select ARCH_INLINE_SPIN_UNLOCK_BH if !PREEMPTION
 	select ARCH_INLINE_SPIN_UNLOCK_IRQ if !PREEMPTION
 	select ARCH_INLINE_SPIN_UNLOCK_IRQRESTORE if !PREEMPTION
+	select ARCH_NEED_CMPXCHG_1_EMU
 	select ARCH_WANT_FRAME_POINTERS if !CPU_CK610 && $(cc-option,-mbacktrace)
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
 	select COMMON_CLK

arch/csky/include/asm/cmpxchg.h

···
 #ifdef CONFIG_SMP
 #include <linux/bug.h>
 #include <asm/barrier.h>
+#include <linux/cmpxchg-emu.h>
 
 #define __xchg_relaxed(new, ptr, size)				\
 ({								\
···
 	__typeof__(old) __old = (old);				\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
+	case 1:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		break;						\
 	case 4:							\
 		asm volatile (					\
 		"1:	ldex.w		%0, (%3) \n"		\
···
 	__typeof__(old) __old = (old);				\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
+	case 1:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		break;						\
 	case 4:							\
 		asm volatile (					\
 		"1:	ldex.w		%0, (%3) \n"		\
···
 	__typeof__(old) __old = (old);				\
 	__typeof__(*(ptr)) __ret;				\
 	switch (size) {						\
+	case 1:							\
+		__ret = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)__ptr, (uintptr_t)__old, (uintptr_t)__new); \
+		break;						\
 	case 4:							\
 		asm volatile (					\
 		RELEASE_FENCE					\