[POWERPC] Fix constantness of bootwrapper arg

Fixes the constantness of the powerpc bootwrapper's console_ops.write
routine.  Allows printing of constant strings.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/boot/serial.c b/arch/powerpc/boot/serial.c
index 7fd3233..eaa0d3a 100644
--- a/arch/powerpc/boot/serial.c
+++ b/arch/powerpc/boot/serial.c
@@ -27,7 +27,7 @@
 	return scdp->open();
 }
 
-static void serial_write(char *buf, int len)
+static void serial_write(const char *buf, int len)
 {
 	struct serial_console_data *scdp = console_ops.data;