irqchip/gic: Move PM device over to irq domain

Move the reference to the GIC device over to the irq domain.
This allows for some localised cleanup.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Link: https://lore.kernel.org/r/20220201120310.878267-3-maz@kernel.org