[POWERPC] Celleb: add celleb_defconfig

This patch creates defconfig file for Celleb platform.
Signed-off-by: NKou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: NArnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>