gpio: mt7621: Make the irqchip immutable

Commit 6c846d02 ("gpio: Don't fiddle with irqchips marked as
immutable") added a warning to indicate if the gpiolib is altering the
internals of irqchips.  Following this change the following warnings
are now observed for the mt7621 driver:

gpio gpiochip0: (1e000600.gpio-bank0): not an immutable chip, please consider fixing it!
gpio gpiochip1: (1e000600.gpio-bank1): not an immutable chip, please consider fixing it!
gpio gpiochip2: (1e000600.gpio-bank2): not an immutable chip, please consider fixing it!

Fix this by making the irqchip in the mt7621 driver immutable.

Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>