gpio: gpio-mm-lantiq: Use of_mm_gpiochip_remove

Since d621e8bae5ac9c67 (Create of_mm_gpiochip_remove), there is a
counterpart for of_mm_gpiochip_add.

This patch implements the remove function of the driver making use of
it.

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