ppc/pnv: add XSCOM handlers to PnvCore

ppc/pnv: add XSCOM handlers to PnvCore

Now that we are using real HW ids for the cores in PowerNV chips, we
can route the XSCOM accesses to them. We just need to attach a
specific XSCOM memory region to each core in the appropriate window
for the core number.

To start with, let's install the DTS (Digital Thermal Sensor) handlers
which should return 38°C for each core.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>