[media] ov2640: improve banding filter register definitions/documentation

- add missing initialisation of sensor register COM25 (2 MSBs of banding
  filter AEC values)
- add macros for setting the banding filter AEC values
- add definitions for sensor register 0x5a, which is documented in
  Omnivisions software application notes

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