media: v4l2-mediabus: add support for dual edge sampling

Some devices support sampling of the parallel data at both edges of the
interface pixel clock in order to reduce the pixel clock by two.
Add a mediabus flag that represents this feature.

Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>