[MIPS] Use "R" constraint for cache_op.

Gcc might emit an absolute address for the the "m" constraint which
gas unfortunately does not permit.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/r4kcache.h

@@ -37,7 +37,7 @@
 	"	cache	%0, %1					\n"	\
 	"	.set	pop					\n"	\
 	:								\
-	: "i" (op), "m" (*(unsigned char *)(addr)))
+	: "i" (op), "R" (*(unsigned char *)(addr)))
 
 static inline void flush_icache_line_indexed(unsigned long addr)
 {