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>