video: of: display_timing: Add support for syncclk-active property

Configure the DISPLAY_FLAGS_SYNC_POSEDGE/NEGEDGE flags according to the
binding document.
If the syncclk-active is present in DT, configure the flags accordingly, if
it is omitted it means that the SYNC edge is following the pixdata
configuration.

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>