iommu: Resolve fwspec ops automatically

There's no real need for callers to resolve ops from a fwnode in order
to then pass both to iommu_fwspec_init() - it's simpler and more sensible
for that to resolve the ops itself. This in turn means we can centralise
the notion of checking for a present driver, and enforce that fwspecs
aren't allocated unless and until we know they will be usable.

Also we've grown a generic fwnode_handle_get() since this code was first
written, so may as well clear up that ugly mismatch while we're in here.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>