devicetree: deprecate DT_PWMS_LABEL APIs

Mark all variants of the DT_PWMS_LABEL APIs:
* DT_PWMS_LABEL_BY_IDX
* DT_PWMS_LABEL_BY_NAME
* DT_PWMS_LABEL
* DT_INST_PWMS_LABEL_BY_IDX
* DT_INST_PWMS_LABEL_BY_NAME
* DT_INST_PWMS_LABEL

As deprecated in favor of utilizing:

* DT_PWMS_CTLR_BY_IDX
* DT_PWMS_CTLR_BY_NAME
* DT_PWMS_CTLR
* DT_INST_PWMS_CTLR_BY_IDX
* DT_INST_PWMS_CTLR_BY_NAME
* DT_INST_PWMS_CTLR

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>