gpio: mockup: use devm_kcalloc() where applicable

When allocating a zeroed array of objects use devm_kcalloc() instead
of manually calculating the required size and using devm_kzalloc().

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>