Add function video_get_video_mode(), which parses the "video-mode" environment
variable and returns each of its components. The format matches the video=
command-line option used for Linux:
video-mode=<driver>:<xres>x<yres>-<depth>@<freq><,option=string>
<driver> The video driver, ignored by U-Boot
<xres> The X resolution (in pixels) to use.
<yres> The Y resolution (in pixels) to use.
<depth> The color depth (in bits) to use.
<freq> The frequency (in Hz) to use.
<options> A comma-separated list of device-specific options
Signed-off-by: Timur Tabi <timur@freescale.com>