dt-bindings: drm/bridge: ti-sn65dsi8: Make enable GPIO optional

The SN65DSI86 EN pin can be hardwired to a high level, or connected to a
global reset signal, not controllable by the kernel. Make it optional in
those cases.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624000304.16281-2-laurent.pinchart+renesas@ideasonboard.com