When using GPIO as IRQ source, the GPIO must be configured
in INPUT. Callbacks dedicated for this was missing in
pinctrl-st driver.

This fix the following kernel error when trying to lock a gpio
as IRQ:

[    7.521095] gpio gpiochip7: (PIO11): gpiochip_lock_as_irq: tried to flag a GPIO set as output for IRQ
[    7.526018] gpio gpiochip7: (PIO11): unable to lock HW IRQ 6 for IRQ
[    7.529405] genirq: Failed to request resources for 0-0053 (irq 81) on irqchip GPIO

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