Input: atmel_mxt_ts - convert bindings to YAML and extend

Input: atmel_mxt_ts - convert bindings to YAML and extend

This converts the Armel MXT touchscreen bindings to YAML format and
extends them with the following two properties:

- vdda-supply: the optional analog supply voltage
- vdd-supply: the optional digital supply voltage

Tested the schema with all in-tree users and they verify fine.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201104153032.1387747-2-linus.walleij@linaro.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>