gpio: stmpe: 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: Patrice Chotard <patrice.chotard@st.com>
Cc: Alexandre TORGUE <alexandre.torgue@st.com>
Link: https://lore.kernel.org/r/20200716100638.112451-1-linus.walleij@linaro.org