gpio: gpio-mm: Implement and utilize register structures

gpio: gpio-mm: Implement and utilize register structures

Reduce magic numbers and improve code readability by implementing and
utilizing named register data structures. The GPIO-MM device features an
Intel 8255 compatible GPIO interface, so the i8255 GPIO module is
selected and utilized as well.

Tested-by: Fred Eckert <Frede@cmslaser.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>