iommu/omap: Remove refcount field from omap_iommu object

The refcount field in omap_iommu object is primarily used to check
if an IOMMU device has already been enabled, but this is already
implicit in the omap_iommu_attach_dev() which ensures that only
a single device can attach to an IOMMU. This field is redundant,
and so has been cleaned up.

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>