iommu: bcm2712-iommu: Allocate tables on demand, not at startup

Allocate space for level-2 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 any unused tables.

Some tidying. Clarify the distinction between Linux pages and MMU
table pages. Increase the default aperture size from 2GB to 4GB.
The 4GB size limit is corrected to (PAGE_SIZE << 20).

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

Author: njhollinghurst