mx51evk: Add DVI output support

Add DVI output support and make it the default video output.

Currently the CLAA WVGA panel is supported, but this panel has to be purchased
separately, so using the DVI output as the default would allow more people to
try the splash screen feature on a mx51evk.

If someone still wants to use the CLAA WVGA, just set the panel variable as:
set panel claa

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>