dt-bindings: pinctrl: convert qcom,mdm9615-pinctrl.txt to dt-schema

Convert the MDM9515 pinctrl bindings to dt-schema.
Keep the parsing of pin configuration subnodes consistent with other Qualcomm
schemas (children named with '-state' suffix, optional children with '-pins').

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20221005-mdm9615-pinctrl-yaml-v2-1-639fe67a04be@linaro.org


[krzk: drop function from required]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>