iommu/omap: Remove unused union fields

There are couple of unions defined in the structures
iotlb_entry and cr_regs. There are no usage/references
to some of these union fields in the code, so clean
them up and simplify the structures.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>