Documentation: ACPI: add new rule for gpio-line-names

The gpio-line-names lists must respect some rules.

This patch adds a new rule in documentation, to avoid
the use of duplicate names in the same gpiochip.
Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/firmware-guide/acpi/gpio-properties.rst

@@ -146,6 +146,7 @@ following rules (see also the examples):
     other words, it is not mandatory to fill all the GPIO lines
   - empty names are allowed (two quotation marks ``""`` correspond to an empty
     name)
+  - names inside one GPIO controller/expander must be unique
 
 Example of a GPIO controller of 16 lines, with an incomplete list with two
 empty names::