Documentation: ACPI: explain how to use gpio-line-names

The "gpio-line-names" declaration is not fully
documented, so can be useful to add some important
information and one more example.

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