Add support for PCIe ARI

Add support for PCIe ARI

PCIe Alternate RID Interpretation (ARI) is an optional feature that
allows devices to have up to 256 different functions.  It is
implemented by always setting the PCI slot number to 0 and
re-purposing the 5 bits used to encode the slot number to instead
contain the function number.  Combined with the original 3 bits
allocated for the function number, this allows for 256 functions.

This is enabled by default, but it's expected to be a no-op on currently
supported hardware.  It's a prerequisite for supporting PCI SR-IOV, and
I want the ARI support to go in early to help shake out any bugs in it.
ARI can be disabled by setting the tunable hw.pci.enable_ari=0.

Reviewed by:	kib
MFC after:	2 months
Sponsored by:	Sandvine Inc.