pinctrl: stm32: Switch to use for_each_gpiochip_node() helper

Switch the code to use for_each_gpiochip_node() helper.

While at it, in order to avoid additional churn in the future,
switch to fwnode APIs where it makes sense.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Fabien Dessenne <fabien.dessenne@foss.st.com>