ack 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

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
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