dt-bindings: display: bridge: sn65dsi83: Make enable GPIO optional

The SN65DSI8x EN signal may be tied to VCC, or otherwise controlled by
means not available to the kernel. Make the GPIO optional.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211213133626.2498056-2-alexander.stein@ew.tq-group.com