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.

Also, there is not an existing pci_adjust_config() similar to
pcie_adjust_config(). I could add that in a separate commit if it
would be useful.

• I use these in a pending change to cxgbe(4), however these are generally useful. We can probably remove some duplicated code in various drivers that uses pci_find_cap() followed by raw pci_read/write_config.