ppc/pnv: change powernv_ prefix to pnv_ for overall naming consistency

The 'pnv' prefix is now used for all and the routines populating the
device tree start with 'pnv_dt'. The handler of the PnvXScomInterface
is also renamed to 'dt_xscom' which should reflect that it is
populating the device tree under the 'xscom@' node of the chip.

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