[POWERPC] CBE thermal support on PS3

I wanted to enable CBE_THERM on PS3.  So I had to enable CBE_RAS first.

But the resulting kernel doesn't link, as cbe_regs.c isn't compiled for
non-PPC_CELL_NATIVE.

CBE_RAS should depend on PPC_CELL_NATIVE; this makes it so.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>