[POWERPC] Always use -mno-string

The string load/store instructions are unimplemented on some processors
and slow (microcoded) on some others.  It's simplest to just not use
them at all.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/Makefile

@@ -102,9 +102,9 @@ CFLAGS += $(call cc-option,-mno-altivec)
 # kernel considerably.
 CFLAGS += $(call cc-option,-funit-at-a-time)
 
-ifndef CONFIG_FSL_BOOKE
-CFLAGS		+= -mstring
-endif
+# Never use string load/store instructions as they are
+# often slow when they are implemented at all
+CFLAGS		+= -mno-string
 
 ifeq ($(CONFIG_6xx),y)
 CFLAGS		+= -mcpu=powerpc