gpiolib: Make gpiochip_hierarchy_add_domain() return domain

As a preparatory patch and for the sake of consistency, make
gpiochip_hierarchy_add_domain() return IRQ domain. While at it,
rename it to gpiochip_hierarchy_create_domain() to show
the change.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>