media: i2c: imx258: Add support for long exposure modes

The sensor has a register CIT_LSHIFT which extends the exposure
and frame times by the specified power of 2 for longer
exposure times.

Add support for this by configuring this register via V4L2_CID_VBLANK
and extending the V4L2_CID_EXPOSURE range accordingly.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>