Documentation: gpio: driver.rst: Remove gpiochip_irqchip_add mention

This function was removed in commit f1f37abb ("gpio: Retire the
explicit gpio irqchip code") but this mention was left behind.
Also, mention that .set_type() only has to set a line handler if the chip
is cascaded, as opposed to hierarchical.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>