pinctrl: stm32: Replace custom code by gpiochip_node_count() call

Since we have generic function to count GPIO controller nodes
under a given device, there is no need to open code it. Replace
custom code by gpiochip_node_count() call.

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