powerpc/mm: Move register_process_table() out of ppc_md

We want to initialise register_process_table() before ppc_md is setup,
so that it can be called as part of MMU init (at least on Radix ATM).

That no longer works because probe_machine() requires that ppc_md be
empty before it's called, and we now do probe_machine() much later.

So make register_process_table a global for now. It will probably move
into a mmu_radix_ops struct at some point in the future.

This was broken by me when applying commit 7025776ed1eb "powerpc/mm:
Move hash table ops to a separate structure" due to conflicts with other
patches.

Fixes: 7025776ed1eb ("powerpc/mm: Move hash table ops to a separate structure")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>