media: omap: omap-iommu.h: allow building drivers with COMPILE_TEST

Drivers that depend on omap-iommu.h (currently, just omap3isp)
need a stub implementation in order to be built with COMPILE_TEST.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>