ipmi: add PNP discovery (ACPI namespace via PNPACPI)

This allows ipmi_si_intf.c to claim IPMI devices described in the ACPI
namespace.  Using PNP makes it simpler to parse the IRQ/IO/memory resources
of the device.

We look at any SPMI tables before looking for devices in the namespace.

This is based on ipmi_pci_probe().

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