gpio: max77620: 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 warning is
now observed for the max77620 gpio driver ...

 WARNING KERN gpio gpiochip0: (max77620-gpio): not an immutable chip,
 	please consider fixing it!

Fix the above warning by making the max77620 gpio driver immutable.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>