media: ov5647: Add V4L2 controls for analogue gain, exposure and AWB

Added basic v4l2_ctrl_handler infrastructure (there was none
previously).

Added controls to let AWB/AEC/AGC run in the sensor's auto mode or
manually. Also controls to set exposure (in lines) and analogue gain
(as a register code) from user code.

Also delete registers (just the one) from the VGA mode register set
that are now controlled by the new V4L2 controls.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Signed-off-by: Naushir Patuck <naush@raspberrypi.com>