pinctrl: stmfx: Fix compile issue when CONFIG_OF_GPIO is not defined

pinctrl: stmfx: Fix compile issue when CONFIG_OF_GPIO is not defined

When CONFIG_GPIO_OF is not defined, struct gpio_chip 'of_node' member does
not exist:
drivers/pinctrl/pinctrl-stmfx.c: In function 'stmfx_pinctrl_probe':
drivers/pinctrl/pinctrl-stmfx.c:652:17: error: 'struct gpio_chip' has no member named 'of_node'
     pctl->gpio_chip.of_node = np;

Fixes: 1490d9f841b1 ("pinctrl: Add STMFX GPIO expander Pinctrl/GPIO driver")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>