gpio: mvebu: Use IS_REACHABLE instead of IS_ENABLED for CONFIG_PWM

To check if a certain function (here e.g. pwmchip_add()) can be called
IS_REACHABLE is the better check. The relevant difference to IS_ENABLED
is that IS_REACHABLE evaluates to 0 if the current code is builtin but the
checked symbol is =m and so must not be used.

Today there is no practical impact as CONFIG_PWM is a bool.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>