PNP: change pnp_add_id() to allocate its own pnp_id structures

This moves some of the pnp_id knowledge out of the backends and into
the PNP core.

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>