pinctrl: equilibrium: Convert to immutable irq_chip

Convert the driver to immutable irq-chip with a bit of
intuition.

Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230403-immutable-irqchips-v1-5-503788a7f6e6@linaro.org