powerpc: Fix __cmpxchg() to take a volatile ptr again

In commit d0563a1297e2 ("powerpc: Implement {cmp}xchg for u8 and u16")
we removed the volatile from __cmpxchg().

This is leading to warnings such as:

  drivers/gpu/drm/drm_lock.c: In function ‘drm_lock_take’:
  arch/powerpc/include/asm/cmpxchg.h:484:37: warning: passing argument 1
  of ‘__cmpxchg’ discards ‘volatile’ qualifier from pointer target
     (__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_,   \

There doesn't seem to be consensus across architectures whether the
argument is volatile or not, so at least for now put the volatile back.

Fixes: d0563a1297e2 ("powerpc: Implement {cmp}xchg for u8 and u16")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/cmpxchg.h

@@ -14,7 +14,7 @@
 #endif
 
 #define XCHG_GEN(type, sfx, cl)				\
-static inline u32 __xchg_##type##sfx(void *p, u32 val)		\
+static inline u32 __xchg_##type##sfx(volatile void *p, u32 val)	\
 {								\
 	unsigned int prev, prev_mask, tmp, bitoff, off;		\
 								\
@@ -40,7 +40,7 @@ static inline u32 __xchg_##type##sfx(void *p, u32 val)		\
 
 #define CMPXCHG_GEN(type, sfx, br, br2, cl)			\
 static inline							\
-u32 __cmpxchg_##type##sfx(void *p, u32 old, u32 new)		\
+u32 __cmpxchg_##type##sfx(volatile void *p, u32 old, u32 new)	\
 {								\
 	unsigned int prev, prev_mask, tmp, bitoff, off;		\
 								\
@@ -399,7 +399,7 @@ __cmpxchg_u64_acquire(u64 *p, unsigned long old, unsigned long new)
 #endif
 
 static __always_inline unsigned long
-__cmpxchg(void *ptr, unsigned long old, unsigned long new,
+__cmpxchg(volatile void *ptr, unsigned long old, unsigned long new,
 	  unsigned int size)
 {
 	switch (size) {