genirq: Kill irq_chip::parent_device

Now that noone is using irq_chip::parent_device in the tree, get
rid of it.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Link: https://lore.kernel.org/r/20220201120310.878267-13-maz@kernel.org