powerpc/Kconfig: select PPC_MM_SLICES from subarch type

Lets select PPC_MM_SLICES from the subarch config item instead of
doing it via defaults declaration in the PPC_MM_SLICES item itself.

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>