iommu: Make kobj_type structure constant

Since commit ee6d3dd4

 ("driver core: make kobj_type constant.")
the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230214-kobj_type-iommu-v1-1-e7392834b9d0@weissschuh.net
Signed-off-by: Joerg Roedel <jroedel@suse.de>