powerpc/mm/radix: Add partition table format & callback

Add structs and #defines related to the radix MMU partition table
format. We also add a ppc_md callback for updating a partition table
entry.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>