The sensor supports either having the CSI2 clock lane free
running, or gated when there is no packet to transmit.
The driver only selected gated (non-continuous) clock mode.

Add code to allow fwnode to configure whether the clock is
gated or free running.
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>