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.orgSigned-off-by: NPali Rohár <pali@kernel.org>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>