soc/arm: stm32: Provide node label based pinctrl devicetree helpers

As an alternative to ST_STM32_DT_INST_PINCTRL, provide
ST_STM32_DT_PINCTRL macro and set of matching internal macros.

This could be used by device drivers that can't use directly
device instances but rather the node label identifier. For instance:
ST_STM32_DT_PINCTRL(0, i2c1);

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>