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

Document the E3 (r8a77990) SoC in the R-Car DU bindings.
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>

Documentation/devicetree/bindings/display/renesas,du.txt

@@ -16,6 +16,7 @@ Required Properties:
     - "renesas,du-r8a77965" for R8A77965 (R-Car M3-N) compatible DU
     - "renesas,du-r8a77970" for R8A77970 (R-Car V3M) compatible DU
     - "renesas,du-r8a77980" for R8A77980 (R-Car V3H) compatible DU
+    - "renesas,du-r8a77990" for R8A77990 (R-Car E3) compatible DU
     - "renesas,du-r8a77995" for R8A77995 (R-Car D3) compatible DU
 
   - reg: the memory-mapped I/O registers base address and length
@@ -63,6 +64,7 @@ corresponding to each DU output.
  R8A77965 (R-Car M3-N)  DPAD 0         HDMI 0         LVDS 0         -
  R8A77970 (R-Car V3M)   DPAD 0         LVDS 0         -              -
  R8A77980 (R-Car V3H)   DPAD 0         LVDS 0         -              -
+ R8A77990 (R-Car E3)    DPAD 0         LVDS 0         LVDS 1         -
  R8A77995 (R-Car D3)    DPAD 0         LVDS 0         LVDS 1         -