To share the IOVA allocator with other architectures, it needs to
accommodate more general aperture restrictions; move the lower limit
from a compile-time constant to a runtime domain property to allow
IOVA domains with different requirements to co-exist.

Also reword the slightly unclear description of alloc_iova since we're
touching it anyway.
Signed-off-by: NRobin Murphy <robin.murphy@arm.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>