LinuxKPI: pci: add more functions

Add a dummy pci_assign_resource() and an implementation of
pci_irq_vector() returning the irq for MSI-X, MSI, and legacy interrupt.
Both are needed by wirless drivers.

Sponsored by: The FreeBSD Foundation
Reviewed by: jhb
Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D38237

(cherry picked from commit fd1a2f3dfc0e8fb20d0d397d586000bb918aab47)
(cherry picked from commit 7b65e6f377ca0005a986c59f1e31a0421d92660f)