dt-bindings: Add binding for Sitronix ST7735R display panels

This adds a new device tree binding for Sitronix ST7735R display panels,
such as the Adafruit 1.8" TFT.

Signed-off-by: David Lechner <david@lechnology.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1514833336-22564-3-git-send-email-david@lechnology.com