iommu/omap: Move data structures to omap-iommu.h

The internal data-structures are scattered over various
header and C files. Consolidate them in omap-iommu.h.

While at this, add the kerneldoc comment for the missing
iommu domain variable and revise the iommu_arch_data name.

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