drivers/gpio: stm32: Enable PM_DEVICE services

Implement power mgmt hooks to support PM_DEVICE and
PM_DEVICE_RUNTIME.
In case of PM_DEVICE_RUNTIME, clock is requested for bank writes
so it is requested before configuring and released only if pin
is not configured as output.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>