PNP: add pnp_get_pnp_resource()

In some places, we need to get the struct pnp_resource, not just
the struct resource, because ISAPNP needs to store the register
index in the pnp_resource.

I don't like pnp_get_pnp_resource() and hope that it is temporary,
but we need it for a little while.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>