video: display_timing: Add flags to select the edge when the sync is driven

The sync can be - and for some panels it must be - driven on different edge
then the data.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: devicetree@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>