PNP: add pnp_new_resource() to find a new unset pnp_resource

This encapsulates the code to locate a new pnp_resource of the
desired type.  Currently this uses the pnp_resource_table, but
it will soon change to find a resource in a linked list.

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