dt-bindings: display: bridge: lvds-transmitter: Document powerdown-gpios

Add documentation for property powerdown-gpios. The property is
optional.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1573660292-10629-3-git-send-email-fabrizio.castro@bp.renesas.com