ARM: OMAP: make iommu subsys_initcall to fix builtin omap3isp

omap3isp depends on omap's iommu and will fail to probe if
initialized before it (which always happen if they are builtin).

Make omap's iommu subsys_initcall as an interim solution until
the probe deferral mechanism is merged.

Reported-by: James <angweiyang@gmail.com>
Debugged-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: stable <stable@vger.kernel.org>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Hiroshi Doyu <hdoyu@nvidia.com>
Cc: Joerg Roedel <Joerg.Roedel@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>