gpiolib: of: remove obsolete comment for of_gpio_get_count()

The function is only being called form the main gpiolib module, so
remove comment saying that it is also used by external callers.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>