MFC 290414,290415:
Additional PCI helper functions.

290414:
Add helper routines for PCI device drivers to read, write, and modify
PCI-Express capability registers (that is, PCI config registers in the
standard PCI config space belonging to the PCI-Express capability
register set).

Note that all of the current PCI-e registers are either 16 or 32-bits,
so only widths of 2 or 4 bytes are supported.

290415:
Add a new helper function for PCI devices to locate the upstream
PCI-express root port of a given PCI device.