PNP: make pnp_add_id() internal to PNP core

pnp_add_id() doesn't need to be exposed outside the PNP core, so
move the declaration to an internal header file.

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>