dt-bindings: pinctrl: qcom,tlmm-common: add common check for function

dt-bindings: pinctrl: qcom,tlmm-common: add common check for function

Certain pins, like SDcard related, do not have functions and such should
not be required.  Add a check for this in common Qualcomm TLMM pin
controller schema.

Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221016170035.35014-8-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>