selftest/gpio: add gpio test case

This test script try to do whitebox testing for gpio subsystem(based on
gpiolib). It manipulate gpio device through chardev or sysfs and check
the result from debugfs. This script test gpio-mockup through chardev by
default. User could test other gpio chip by passing the module name.
Some of the testcases are turned off by default to avoid the conflicting
with gpiochip in system.

In details, it test the following things:
1.  Test direction and output value for valid pin.
2.  Test dynamic allocation of gpio base.
3.  Add single, multi gpiochip to do overlap check.

Run "tools/testing/selftests/gpio/gpio-mockup.sh -h" for usage.

Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>