iommu/omap: Allow enable/disable even without pdata

When booting with a devicetree, no platform data is provided.
Do not prematurely exit iommu_enable() and iommu_disable() in
such a case.

Note: As OMAP do not yet has a proper reset controller driver,
IOMMUs requiring a reset signal should use pdata-quirks as a
transitional solution.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>