pinctrl: stm32: Handle RSVD pin configuration

pinctrl: stm32: Handle RSVD pin configuration

When a pin is assigned to a coprocessor not running Linux, the pin
should be flagged in the DT as reserved and not usable by Linux.

Add pin configuration RSVD (Reserved) and instrument the driver to
not update the mux setting.

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Link: https://lore.kernel.org/20250610152309.299438-2-antonio.borneo@foss.st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>