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>