gpiolib: update inline documentation of gpiod_get_index()

The patch just accents that @dev could be NULL.

There is no functional change.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -2423,7 +2423,7 @@ EXPORT_SYMBOL_GPL(gpiod_get);
 
 /**
  * gpiod_get_index - obtain a GPIO from a multi-index GPIO function
- * @dev:	GPIO consumer
+ * @dev:	GPIO consumer, can be NULL for system-global GPIOs
  * @con_id:	function within the GPIO consumer
  * @idx:	index of the GPIO to obtain in the consumer
  *