Add 720x576 (DV-PAL) as another fallback v4l2 framesize

I added 720x576 (DV-PAL) as another fallback framesize.
The device, I am currently testing with, is using this framesize, which is why I added it.
I do not know if it is worth adding to OBS-studio, if not, I am also ok with changing it just for me.

Datasheet as reference: http://www.terratec.com/media/filer_public/10599-en.pdf