[POWERPC] celleb: Move the files for celleb base support

This moves the base code for celleb support into platforms/cell/.
All files in this patch are used by celleb-beat and celleb-native
commonly.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>