[media] ov2640: add information about DSP register 0xc7

According to ov2640 software application notes, there are two Automatic
White Balance (AWB) modes, which are selected by DSP register 0xc7:

1) Simple AWB: assumes the average color is gray
   + independent from lens
   - doesn't work well if captured area contains unbalanced colors
     (e.g. large blue background)

2) Advanced AWB: uses color temperature information
   + more accurate, works with all image contents
   - lens specific, requires calibration

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>