pinctrl: at91: Make the irqchip immutable

To help gpiolib not fiddle around with the internals of the irqchip
flag the chip as immutable, adding the calls into the gpiolib core
required to do so.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230216-gpio-at91-immutable-v2-1-326ef362dbc7@kernel.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>