dt-bindings: display: panel: Convert sitronix,st7701 to DT schema

Convert the sitronix,st7701 panel bindings to DT schema.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403142453.25307-2-jagan@amarulasolutions.com