dt-bindings: display: renesas: lvds: Document r8a77990 bindings

The E3 (r8a77990) supports two LVDS channels. Extend the binding to
support them.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

Documentation/devicetree/bindings/display/bridge/renesas,lvds.txt

@@ -15,6 +15,7 @@ Required properties:
   - "renesas,r8a7796-lvds" for R8A7796 (R-Car M3-W) compatible LVDS encoders
   - "renesas,r8a77970-lvds" for R8A77970 (R-Car V3M) compatible LVDS encoders
   - "renesas,r8a77980-lvds" for R8A77980 (R-Car V3H) compatible LVDS encoders
+  - "renesas,r8a77990-lvds" for R8A77990 (R-Car E3) compatible LVDS encoders
   - "renesas,r8a77995-lvds" for R8A77995 (R-Car D3) compatible LVDS encoders
 
 - reg: Base address and length for the memory-mapped registers