PCI: Document ACPI description of PCI host bridges

Add a writeup about how PCI host bridges should be described in ACPI
using PNP0A03/PNP0A08 devices, PNP0C02 devices, and the MCFG table.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>