[POWERPC] Make xmon_write accept a const buffer

Because xmon_write doesn't change the buffer, we should add 'const'
qualifier to the argument which points it.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/xmon/start.c b/arch/powerpc/xmon/start.c
index 712552c..8864de2 100644
--- a/arch/powerpc/xmon/start.c
+++ b/arch/powerpc/xmon/start.c
@@ -14,7 +14,7 @@
 {
 }
 
-int xmon_write(void *ptr, int nb)
+int xmon_write(const void *ptr, int nb)
 {
 	return udbg_write(ptr, nb);
 }