gpio: mockup: implement gpio_mockup_set_multiple()

Implement the set_multiple() callback and register it with the gpiolib
framework. This is only meant to also test the internal kernel API.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>