media: i2c: imx290: Use runtime PM autosuspend

Use runtime PM autosuspend to avoid powering off the sensor during fast
stop-reconfigure-restart cycles. This also fixes runtime PM handling in
the probe function that didn't suspend the device, effectively leaving
it resumed forever.

While at it, improve documentation of power management in probe() and
remove().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>