gpio: pcf857x: 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 pcf857x driver:

    gpio gpiochip1: (pcf8575): not an immutable chip, please consider fixing it!

Fix this by making the irqchip in the pcf857x driver immutable.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>