ipa: ipu3: awb: Introduce Black Level Correction

The pixels output by the camera normally include a black level, because
sensors do not always report a signal level of '0' for black. Pixels at
or below this level should be considered black and to achieve that, we
need to substract an offset to all the pixels. This can be taken into
account by reading the lowest value of a special region on sensors which
is not exposed to light. This provides a substracting factor to be
able to adjust the expected black levels in the resulting images.

For a camera outputting 10-bit pixel values (in the range 0 to 1023) a
typical black level might be 64. It is a fixed value, obtained by
capturing a raw frame with minimum exposure and gain fixed to 1.0 while
covering the sensor (the darker the better). We consider it good enough
as a very first approximation, until we measure it during a tuning
process and include it in a configuration file

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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