iommu: bcm2712-iommu: Mke IOVA aperture base and size configurable

Add OF parameters to override the default aperture base (40GB)
and size (2GB). These parameters do not include dma_iova_offset.

This means that some table-size calculations are no longer static.

At the same time, rearrange how aperture is represented internally
so that it *does* include dma_iova_offset, since all DMA addresses
shared with other drivers (e.g. on RP1) will have this offset added.

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

Author: njhollinghurst