[POWERPC] Fix oprofile support on Cell LPAR

Op_model_cell supports native Cell. By returning -EINVAL, oprofile
uses timer interrupt on Cell LPAR.

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