target/ppc: Add one reg id for ptcr

The ptcr (partition table control register) is used to store the address
and size of the partition table. For nested kvm-hv we have a level 1
guest register the location of it's partition table with the hypervisor.
Thus to support migration we need to be able to read this out of kvm
and restore it post migration.

Add the one reg id for the ptcr.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>