powerpc/powernv: Set the PTCR for the Nest MMU

The Nest MMU manages address translation for accelerators on the POWER9. To
do so, it needs a page table, so export the system page table to the Nest
MMU. This will quietly fail on pre-POWER9 systems that do not have a NMMU.

The NMMU is currently unused, so this change is currently effectively a NOP,
but the NMMU and VAS will eventually be used.