iommu/omap: Fix map protection value handling

The prot flags passed to the IOMMU map handler are defined in
include/linux/iommu.h as IOMMU_(READ|WRITE|CACHE|EXEC). However, the
driver expects to receive MMU_RAM_* OMAP-specific flags. This causes
IOMMU flags being interpreted as page sizes, leading to failures.

Hardcode the OMAP mapping parameters to little-endian, 8-bits and
non-mixed page attributes. Furthermore, as the OMAP IOMMU doesn't
support read-only or write-only mappings, ignore the prot value.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Suman Anna <s-anna@ti.com>