PCI: mvebu: Disallow mapping interrupts on emulated bridges

Interrupt support on mvebu emulated bridges is not implemented yet.

So properly indicate return value to callers that they cannot request
interrupts from emulated bridge.

Link: https://lore.kernel.org/r/20211125124605.25915-6-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>