pinctrl: stm32: add lock mechanism for irqmux selection

GPIOs are split between several banks (A, B, ...) and each bank can have
up to 16 lines. Those GPIOs could be used as interrupt lines thanks to
exti lines. As there are only 16 exti lines, a mux is used to select which
gpio line is connected to which exti line. Mapping is done as follow:

-A0, B0, C0.. -->exti_line_0 (X0 selected by mux_0)
-A1, B1, C1.. -->exti_line_1 (X1 selected by mux_1)
...

This patch adds a protection to avoid overriding on mux_n for exti_line_n.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>