iommu/dma: Allow drivers to reserve an iova range

Some devices have a memory map which contains gaps or holes. In order for
the device to have as much iova space as possible, allow its driver to
inform the dma-iommu that it should not allocate addresses from these
holes.

Change-Id: I029d50c1f1e84ece48de30ef21b63cac8fe05257
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>