[media] v4l: of: Read lane-polarities endpoint property

Add lane_polarities field to struct v4l2_of_bus_mipi_csi2 and write the
contents of the lane-polarities property to it. The field tells the polarity
of the physical lanes starting from the first one. Any unused lanes are
ignored, i.e. only the polarity of the used lanes is specified.

Also rework reading the "data-lanes" property a little.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>