dt-bindings: panel: lvds: Document power-supply property

The power-supply property is used by a vast majority of panels, including
panel-simple. Let's document it as a common property

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0a6a3abcf1a6b7f0e66a81af8a44c5c0566ce06c.1513854122.git-series.maxime.ripard@free-electrons.com