pinctrl: stmfx: Use irqchip template

This makes the driver use the irqchip template to assign
properties to the gpio_irq_chip instead of using the
explicit calls to gpiochip_irqchip_add_nested() and
gpiochip_set_nested_irqchip(). The irqchip is instead
added while adding the gpiochip.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Link: https://lore.kernel.org/r/20200721131814.357182-1-linus.walleij@linaro.org