gpio: sifive: 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 sifive driver:

    gpio gpiochip1: (38001000.gpio-controller): not an immutable chip, please consider fixing it!

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

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>