dt-bindings: input: tm2-touchkey: convert to yaml

Convert the device tree bindings for tm2-touchkey to the YAML format.

While we're at it, clarify the descriptions a bit to make it clear that
this driver can be used for many different MCUs that all implement
a similar I2C protocol. Depending on the MCU the voltage requirements
may be different, on some devices the controller uses 2.2V, 2.8V or
even 3.3V for vcc-supply instead of 1.8V.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201203131242.44397-1-stephan@gerhold.net
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>