ARM: dts: imx51-babbage: Fix the DVI output description

imx51-babbage has a TFP410 chip that receives 24-bit RGB parallel
input and convert it to DVI.

Fix the device tree description to reflect the real hardware.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>