irqchip/imx-intmux: Move PM device over to irq domain

Move the reference to the device over to the irq domain.
This allows the irq_chip structure to be directly used instead
of taking a copy for each instance.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Link: https://lore.kernel.org/r/20220201120310.878267-6-maz@kernel.org