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.

Reviewed by: imp
MFC after: 1 week
Sponsored by: Chelsio
Differential Revision: https://reviews.freebsd.org/D4088