pinctrl: stm32: move gpio irqs binding to optional

stm32 pinctrl driver could be probed even if no interrupt controller
is defined to manage gpio irqs. Entries related to gpio irq management
are moved to optional.

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