OMAP: iommu: make iva2 iommu selectable

It seems dsp-link will do this, and tidspbridge too at some point, but
right now it's not possible to select CONFIG_MPU_BRIDGE_IOMMU.

Cc: Fernando Guzman Lugo <fernando.lugo@ti.com>
Cc: Yogesh Marathe <yogesh_marathe@ti.com>
Signed-off-by: Felipe Contreras <felipe.contreras@nokia.com>