media: i2c: imx477: Add very long exposure control to the driver

Add support for very long exposures by using the exposure multiplier
register. Userland does not need to pass any additional controls to
enable long exposures, it simply requests a larger vblank to extend the
exposure control range appropriately.

Currently, since hblank is fixed, a maximum of approximately 124 seconds
of exposure time can be used. In a future change, hblank could also be
controlled in userland to give over 200 seconds of exposure time.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>