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>

Documentation/driver-api/gpio/driver.rst

@@ -547,13 +547,10 @@ To use the helpers please keep the following in mind:
   the irqchip can initialize. E.g. .dev and .can_sleep shall be set up
   properly.
 
-- Nominally set all handlers to handle_bad_irq() in the setup call and pass
-  handle_bad_irq() as flow handler parameter in gpiochip_irqchip_add() if it is
-  expected for GPIO driver that irqchip .set_type() callback will be called
-  before using/enabling each GPIO IRQ. Then set the handler to
-  handle_level_irq() and/or handle_edge_irq() in the irqchip .set_type()
-  callback depending on what your controller supports and what is requested
-  by the consumer.
+- Nominally set gpio_irq_chip.handler to handle_bad_irq. Then, if your irqchip
+  is cascaded, set the handler to handle_level_irq() and/or handle_edge_irq()
+  in the irqchip .set_type() callback depending on what your controller
+  supports and what is requested by the consumer.
 
 
 Locking IRQ usage