[POWERPC] Update Cell MAINTAINERS entry, add spufs entry

The MAINTAINERS file entry for the cell platform is outdated,
even the name of the platform changed since the early days
when it was initially submitted.
The SPU file system is now maintained by Jeremy Kerr.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>