pinctrl: ocelot: Make irq_chip immutable

Since recently, the kernel is nagging about mutable irq_chips:

[    2.593426] gpio gpiochip0: (ocelot-gpio): not an immutable chip, please consider fixing it!

Make it const, flag it as IRQCHIP_IMMUTABLE, add the new helper
functions and call the appropriate gpiolib functions.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220706151553.1580790-2-michael@walle.cc