dt-bindings: display: panel-simple-dsi: add optional reset gpio

Simple DSI panels can also have a reset GPIO signal in addition/instead of an
enable GPIO signal.

This adds an optional reset-gpios property.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915121912.4347-3-narmstrong@baylibre.com