pinctrl: stm32: activate strict mux mode

This activates strict mode muxing for the STM32 pin controllers,
as these do not allow GPIO and functions to use the same pin
simultaneously.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>