powerpc/mm: Move hash table ops to a separate structure

Moving probe_machine() to after mmu init will cause the ppc_md
fields relative to the hash table management to be overwritten.

Since we have essentially disconnected the machine type from
the hash backend ops, finish the job by moving them to a different
structure.

The only callback that didn't quite fix is update_partition_table
since this is not specific to hash, so I moved it to a standalone
variable for now. We can revisit later if needed.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Fix ppc64e build failure in kexec]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>