iommu: bcm2712-iommu: Allocate tables on demand; add OF properties

Allocate space for level-2 IOMMU translation tables as needed.
This should save memory in most cases but it means that map()
can now fail with -ENOMEM. We don't currently free unused tables.

This let us increase the default aperture size from 2GB to 4GB.
Add OF properties to override the default aperture size and base
address (where aperture-base does not include dma-iova-offset).

Various tidy-ups. The internal representation of aperture limits
does include dma_iova_offset, as that is more commonly useful.
Clarify the distinction between Linux pages and IOMMU table pages.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>

Author: njhollinghurst