gpio: gpio-reg: fix build

Revert changes introduced by commit f0fbe7bc ("gpio: Move irqdomain
into struct gpio_irq_chip") as they are not aplicable to this driver.

Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Fixes: f0fbe7bc ("gpio: Move irqdomain into struct gpio_irq_chip")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>