gpio: stp-xway: allow compile-testing

Enable compile-testing of the stp-xway GPIO driver now that it does not
depend on any architecture specific includes anymore.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20190702223248.31934-5-martin.blumenstingl@googlemail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>