powerpc/mm: Make some of the PGTABLE_RANGE dependency explicit

slice array size and slice mask size depend on PGTABLE_RANGE.

Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>