pinctrl: st: Add irq_disable hook to st_gpio_irqchip

Currently disable_irq() doesn't work for pinctrl-st driver, due to
missing irq_disable hook in the driver.
disable_irq() is required only for level-triggered interrupts, which
is not the case normally.

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