dt-bindings: display: bridge: lvds-codec: Document power-supply property

Document optional power-supply property that may be used to specify the
regulator powering up the device.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>