[POWERPC] fix showing xmon help

In some configuration, xmon help string is larger than xmon_printf
buffer.  We need not to use printf.  This patch adds xmon_puts and
change to use it to show help string.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 669e656..121b04d 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -833,7 +833,7 @@
 			mdelay(2000);
 			return cmd;
 		case '?':
-			printf(help_string);
+			xmon_puts(help_string);
 			break;
 		case 'b':
 			bpt_cmds();