PCI: microchip: Include <linux/irqdomain.h> explicitly

pcie-microchip-host.c uses irq_domain_add_linear() and related interfaces,
so it needs <linux/irqdomain.h> but doesn't include it directly; it relies
on the fact that <linux/of_irq.h> includes it.

But pcie-microchip-host.c *doesn't* need <linux/of_irq.h> itself.  Include
<linux/irqdomain.h> directly to remove this implicit dependency so a future
patch can drop <linux/of_irq.h>.

Link: https://lore.kernel.org/r/20221031153954.1163623-3-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>