[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>

arch/powerpc/boot/ofconsole.c

@@ -33,7 +33,7 @@ static int of_console_open(void)
 	return -1;
 }
 
-static void of_console_write(char *buf, int len)
+static void of_console_write(const char *buf, int len)
 {
 	of_call_prom("write", 3, 1, of_stdout_handle, buf, len);
 }