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

Allocate space for level-2 IOMMU translation tables on demand.
This should save memory in most cases but means that map_pages()
can now fail with -ENOMEM. Unused pages are retained for re-use.

Add OF properties to override the default aperture size (2GB)
and base address (40GB); this doesn't include any 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.
Fix wrong definition of MMMU_CTRL_PT_INVALID_EN flag.

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

Author: njhollinghurst