PNP: add CONFIG_PNP_DEBUG_MESSAGES and pnp_dbg()

This adds the core function pnp_dbg() and a new config option to
enable it.

The PNP core debugging messages can be enabled at boot-time with the
"pnp.debug" kernel parameter.

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