pinctrl: stm32: fix PM support for stm32mp257

The driver for stm32mp257 is missing the suspend callback in
struct dev_pm_ops.

Add the callback, using the common stm32_pinctrl_suspend()
function.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Fixes: 619f8ca4 ("pinctrl: stm32: add stm32mp257 pinctrl support")
Link: https://lore.kernel.org/r/20240207104604.174843-1-antonio.borneo@foss.st.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>