[POWERPC] Celleb: Serial I/O update

This is an update for Serial I/O on Celleb.
  - Detection algorithm has been changed

Signed-off-by: Kou Ishizaki <Kou.Ishizaki@toshiba.co.jp>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>