[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>