gpio: kona: enable only on BCM_MOBILE or for compile testing

This change makes it easier to configure a kernel for a real machine by
not showing the option to enable it at all if COMPILE_TEST is off.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>