gpio: realtek-otto: Make the irqchip immutable

Since commit 6c846d02 ("gpio: Don't fiddle with irqchips marked as
immutable") a warning is issued for the realtek-otto driver:

    gpio gpiochip0: (18003500.gpio): not an immutable chip, please consider fixing it!

Make the driver's irqchip immutable to fix this.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>