[POWERPC] Fix celleb sio section warning

Fix following warning:
WARNING: vmlinux.o(.text+0x45fd4): Section mismatch: reference to .init.text:.early_serial_txx9_setup (between '.txx9_serial_init' and '.txx9_serial_config')
Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/platforms/celleb/scc_sio.c

@@ -39,7 +39,7 @@ static struct {
 	{ 0x800, 1 }	/* 0xFF2800 */
 };
 
-static int txx9_serial_init(void)
+static int __init txx9_serial_init(void)
 {
 	extern int early_serial_txx9_setup(struct uart_port *port);
 	struct device_node *node;