pci_cfgreg: Add a PCI domain argument to the low-level register API

This commit changes the API of pci_cfgreg(read|write) to add a domain
argument (referred to as a segment in ACPI parlance) (note that this
is not the same as a NUMA domain, but something PCI-specific). This
does not yet enable access to domains other than 0, but updates the
API to support domains.

Places that use hard-coded bus/slot/function addresses have been
updated to hardcode a domain of 0. A few places that have the PCI
domain (segment) available such as the acpi_pcib_acpi.c Host-PCI
bridge driver pass the PCI domain.

The hpt27xx(4) and hptnr(4) drivers fail to attach to a device not on
domain 0 since they provide APIs to their binary blobs that only
permit bus/slot/function addressing.

The x86 non-ACPI PCI bus drivers all hardcode a domain of 0 as they do
not support multiple domains.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D42827