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 <inochiama@outlook.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/IA1PR20MB4953C71C87FD150D2E64FF41BBAC2@IA1PR20MB4953.namprd20.prod.outlook.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>