dt-bindings: display: sitronix,st7735r: Convert to DT schema

Convert the DT binding documentation for Sitronix ST7735R displays to DT
schema.

Add a reference to the Adafruit 1.8" LCD while at it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115124548.3951-2-geert+renesas@glider.be