s5k3l6xx: V4L2 controls tweaks

Added digital gain, set analog gain's minimum value to 0x20 per the datasheet, and prevented analog gain and integration time from being reset by frame mode register sets.

@dorota.czaplejewicz In the end I haven't implemented the exposure control since I'm not exactly sure how to map a single V4L2_CID_EXPOSURE value into coarse and fine integration times, and also what should be the range of such control (testing shows that somewhere around 4000 values stop influencing the image anymore)