Add basic gpio operations. User could get/set gpio value for specific
line of gpiochip.

Reference "tools/gpio/gpio-hammer.c" or
"tools/testing/selftest/gpio/gpio-mockup-chardev.c" for how to use it.
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Reviewed-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>