iommu/shmobile: Allocate archdata with kzalloc()

The archdata attached_list field isn't initialized, leading to random
crashes when accessed. Use kzalloc() to allocate the whole structure and
make sure all fields get initialized properly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>