dt-bindings: display: display-timing: Add property to configure sync drive edge

There are display panels which demands that the sync signal is driven on
different edge than the pixel data.
With the syncclk-active property we can specify the clk edge to be used to
drive the sync signal. When the property is missing it indicates that the
sync is driven on the same edge as the pixel 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
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>