pinctrl: pinmux: Add missing .function_is_gpio kerneldoc

This callback was undocumented, add the docs.

Reviewed-by: Bartosz Golaszewski <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

include/linux/pinctrl/pinmux.h

···
  *	name can be used with the generic @pinctrl_ops to retrieve the
  *	actual pins affected. The applicable groups will be returned in
  *	@groups and the number of groups in @num_groups
+ * @function_is_gpio: determine if the indicated function selector passed
+ *	corresponds to the GPIO function which is used by the accelerated GPIO
+ *	functions @gpio_request_enable, @gpio_disable_free and
+ *	@gpio_set_direction. When the pin control core can properly determine
+ *	if a function is a GPIO function, it is easier to use the @strict mode
+ *	on the pin controller. Since a single function is passed, this is
+ *	only useful on pin controllers that use a specific function for GPIO,
+ *	and that usually presupposes that a one-group-per-pin approach is
+ *	used, so that a single function can be set on a single pin to turn
+ *	it to GPIO mode.
  * @set_mux: enable a certain muxing function with a certain pin group. The
  *	driver does not need to figure out whether enabling this function
  *	conflicts some other use of the pins in that group, such collisions