dt-bindings: input: tm2-touchkey: document vddio-supply

The Samsung touchkey controllers are often used with external pull-up
for the interrupt line and the I2C lines, so we might need to enable
a regulator to bring the lines into usable state. Otherwise, this might
cause spurious interrupts and reading from I2C will fail.

Document support for a "vddio-supply" that is enabled by the tm2-touchkey
driver so that the regulator gets enabled when needed.

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/20201203131242.44397-2-stephan@gerhold.net
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>