Implement exposure controls

I believe the situation is the same for both sensors, with coarse/fine integration time being expressed in pixels, and pixel time depending on some clock timing.

From https://www.kernel.org/doc/html/v4.10/media/uapi/v4l/extended-controls.html#camera-control-ids

V4L2_CID_EXPOSURE_ABSOLUTE (integer) Determines the exposure time of the camera sensor. The exposure time is limited by the frame interval. Drivers should interpret the values as 100 µs units, where the value 1 stands for 1/10000th of a second, 10000 for 1 second and 100000 for 10 seconds.

So some calculation needs to be done kernel-side, and it seems frame interval needs to be correctly exposed for it to be useable.

The hard parts:

• calculating the actual clock rate
• find out which clock is responsible for the pixel/line timings (it's not the MIPI clock, is it?).

At least on the Samsung sensor, information coming from the hard parts will be needed for designing longer frame time modes.