ppc/pnv: populate device tree for RTC devices

The code could be common to any ISA device but we are missing the IO
length.

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