video: mxsfb: Add an entry for mx23evk/mx28vk video modes

Currently the mxsfb driver takes the display timings from the 'videomode'
environment variable.

Provide an example on how to set 'videomode' for using splash screen on
mx23evk and mx28vk boards.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Marek Vasut <marex@denx.de>