gpiolib: Factor out gpiochip_simple_create_domain()

As a preparatory patch and for the sake of consistency,
factor out gpiochip_simple_create_domain().

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>