soc/arm: stm32: Clean up pinctrl devicetree helper macros

Before adding new macros, clean up existing file:
* Provide full length description for each macro
* Cascade the device instance pinctl- property index so that
any pinctrl- instance number could be used
* Remove intermediate ST_STM32_DT_PIN macro

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