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>

diff --cc drivers/gpio/gpiolib-of.c
Simple merge

diff --cc include/linux/of_gpio.h
Simple merge