video: support YUYV fourcc as alias for YUY2

Submitted by Aleix Conchillo Flaqué

Link to original bug (#745639)

Description

Calling gst_video_format_from_fourcc with YUYV returns GST_VIDEO_FORMAT_UNKNOWN,

YUYV is an alias for YUY2 so it should return YUY2 safely.

I've found this by reading the pixelformat from v4l2.