LinuxKPI: extend pci.h by various functions for wireless driver

Add dummy functions for dealing with "HotPlug" events which we currently
do not support.

Add pci_dev_get(), pci_find_ext_capability() and pci_pme_capable().

The added pcie_find_root_port() is a bit special as we need to create
another linux pci device; for that make lkpinew_pci_dev() public
which is also helpful for other cases when we want to use the Linux
routines to check for device identifiers only and need a container
for the "bsddev" to use natively. This has proven to avoid basic
checking code for the sake of rewriting it to native field names
elsewhere. Given we cache the newly created "root" we also need to
make sure we clean it up.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30521