arm64/its: Support devices not behind an iommu

When the IOMMU option is enabled but a device doesn't have an IOMMU
this will return an error. If the device is really behind an IOMMU
then it will fail later, and if not this is needed to allocate MSI and
MSI-X interrupts.

Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48725