staging: imx-drm: add LVDS666 support for parallel display

Support the LVDS666 format on the IPUv3 parallel display.
This makes the screen work on my Hercules eCAFE Slim HD.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>