gpio: sim: new testing module

Implement a new, modern GPIO testing module controlled by configfs
attributes instead of module parameters. The goal of this driver is
to provide a replacement for gpio-mockup that will be easily extensible
with new features and doesn't require reloading the module to change
the setup.

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