[media] mt9p031: Implement black level compensation control
Add four new controls to configure black level compensation (BLC):
- V4L2_CID_BLC_AUTO selects between manual and auto BLC
- V4L2_CID_BLC_TARGET_LEVEL sets the target level for auto BLC
- V4L2_CID_BLC_ANALOG_OFFSET sets the analog offset for manual BLC
- V4L2_CID_BLC_DIGITAL_OFFSET sets the digital offset for manual BLC