selftests: remove obsolete build restriction for gpio

Build restrictions related to the gpio-mockup-chardev helper are
no longer relevant so remove them.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>