media: v4l: fwnode: Support default CSI-2 lane mapping for drivers

Most hardware doesn't support re-mapping of the CSI-2 lanes. Especially
sensor drivers have a default number of lanes. Instead of requiring the
caller (the driver) to provide such a unit mapping, provide one if no
mapping is configured.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>