dt-bindings: display: bridge: lvds-transmitter: Document powerdown-gpios

Add documentation for property powerdown-gpios. The property is
optional.
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1573660292-10629-3-git-send-email-fabrizio.castro@bp.renesas.com

Documentation/devicetree/bindings/display/bridge/lvds-transmitter.yaml

@@ -57,6 +57,11 @@ properties:
       - port@0
       - port@1
 
+  powerdown-gpios:
+    description:
+      The GPIO used to control the power down line of this device.
+    maxItems: 1
+
 required:
   - compatible
   - ports