Add OFW support to arm64's IOMMU framework.
This is needed to support non-PCI devices like memory-mapped
display controllers.
Split-out some initialization code from iommu_ctx_alloc() into
iommu_ctx_init() method so we could pass controller's MD-data
obtained from DTS to the driver prior to a CTX initialization.

Tested on Morello SoC.

Sponsored by: UKRI