powerpc/mm: Increase the slice range to 64TB

This patch makes the high psizes mask as an unsigned char array
so that we can have more than 16TB. Currently we support upto
64TB

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>