drivers: video: Make VIDEO_FOURCC() a documented API

Rename video_fourcc() to VIDEO_FOURCC(), differing from the Linux
implementation, but more compliant with the coding style.

Also introduce a VIDEO_FOURCC_FROM_STR() to generate a FOURCC format
code out of a 4-characters string.

Signed-off-by: Josuah Demangeon <me@josuah.net>