dt-bindings: pincfg-node: Add "input-schmitt-microvolt" property

On Sophgo CV18XX platform, threshold strength of schmitt trigger can
be configured. As this standard property is already supported by the
common pinconf code. Add "input-schmitt-microvolt" property in
pincfg-node.yaml so that other platforms requiring such feature can
make use of this property.

Signed-off-by: Inochi Amaoto <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/pinctrl/pincfg-node.yaml

···
     type: boolean
     description: disable schmitt-trigger mode
 
+  input-schmitt-microvolt:
+    description: threshold strength for schmitt-trigger
+
   input-debounce:
     $ref: /schemas/types.yaml#/definitions/uint32-array
     description: Takes the debounce time in usec as argument or 0 to disable