gpio: mockup: merge gpio_mockup_add() into gpio_mockup_probe()

Now that the probe() function only does what is should, there's no
need to split the chip adding logic into a separate routine. Merge
gpio_mockup_add() into gpio_mockup_probe().

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