pinctrl: mcp23s08: Use irqchip template

pinctrl: mcp23s08: 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>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Phil Reid <preid@electromag.com.au>
Cc: Lars Poeschel <poeschel@lemonage.de>
Cc: Jason Kridner <jkridner@gmail.com>
Link: https://lore.kernel.org/r/20200721125223.344411-1-linus.walleij@linaro.org