Document the optional clock-noncontinuous endpoint property that
allows enabling MIPI CSI-2 non-continuous clock operations.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>