[POWERPC] update cell_defconfig

Sync with the Kconfig changes, and enable some options for celleb

Cc: Ishizaki Kou <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Jens Osterkamp <jens@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>