pinctrl: starfive: 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 starfive driver:

    gpio gpiochip0: (11910000.pinctrl): not an immutable chip, please consider fixing it!

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

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/5eb66be34356afd5eb0ea9027329e0939d03d3a0.1652884852.git.geert+renesas@glider.be


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