selftests: gpio: add config fragment for gpio-mockup

The gpio test requires to insert the gpio mockup module (CONFIG_GPIO_MOCKUP).
The gpio mockup driver depends on gpiolib (CONFIG_GPIOLIB).
CONFIG_GPIO_SYSFS is selected automatically by the gpio mockup driver.

Tested on x86_64 and arm64 with defconfig and kselftest-merge.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>