Documentation: ACPI: enumeration: add PCI hierarchy representation

For "fixed" PCI devices, such as chips directly soldered
on the main board (ethernet, Wi-Fi, serial ports, etc.),
it is possible to find an ACPI enumeration.

This allows to add useful properties to these devices.
Just for an example: the property "gpio-line-names" can be
added to the pins of a GPIO expander on the PCI bus.

In order to find the ACPI name of a PCI device, it's necessary
to disassemble the BIOS ACPI tables (in particular the DSDT)
and also to analyze the PCI bus topology of the board.

This patch, with a practical example, show how to do this.

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>