pinctrl: st: Switch to use fwnode instead of of_node

pinctrl: st: Switch to use fwnode instead of of_node

The OF node in the GPIO library is deprecated and soon
will be removed.

GPIO library now accepts fwnode as a firmware node, so
switch the driver to use it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20221010075615.43244-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>