dt-bindings: input/touchscreen: add bindings for msg2638

This adds dts bindings for the mstar msg2638 touchscreen.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vincent Knecht <vincent.knecht@mailoo.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210305153815.126937-1-vincent.knecht@mailoo.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>