gpio: pcf857x: Switch to use gpiolib irqchip helpers

Switch the PCF857x GPIO driver to use the gpiolib irqchip helpers.
This driver uses a nested threaded interrupt, hence handle_nested_irq()
and gpiochip_set_chained_irqchip() must be used.

Note that this removes the checks added in commit 21fd3cd1


("gpio: pcf857x: call the gpio user handler iff gpio_to_irq is done"),
as the interrupt mappings are no longer created on-demand by the driver,
but by gpiochip_irqchip_add() during initialization.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>