video: parse the video-mode environment variable

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>