dt-bindings: add binding for Sitronix ST7586 display panels

This adds a new binding for Sitronix ST7586 display panels.

Using lego as the vendor prefix in the compatible string because the display
panel I am working with is an integral part of the LEGO MINDSTORMS EV3.

Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1502127581-10517-3-git-send-email-david@lechnology.com