gpio/gpiolib-of: Create of_mm_gpiochip_remove

Create counterpart of of_mm_gpiochip_add(). This way the modules that
can be removable do not duplicate the cleanup code.

Suggested-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>