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 <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/devicetree/bindings/input/cypress,tm2-touchkey.yaml

···
   vcc-supply:
     description: Optional regulator for MCU, 1.8V-3.3V (depending on MCU).
 
+  vddio-supply:
+    description: |
+      Optional regulator that provides digital I/O voltage,
+      e.g. for pulling up the interrupt line or the I2C pins.
+
   linux,keycodes:
     minItems: 1
     maxItems: 4