rtc: dont reference pnp_resource_table directly

pnp_resource_table is going away soon, so use the more
generic public interfaces instead.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-By: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>