drivers: gpio: stm32: improve initialization

Call `gpio_stm32_clock_request` unconditionally with "true" if no
runtime PM is enabled, "false" otherwise. In case the GPIO was enabled
(e.g. by a bootloader), the device will be left in proper state when
runtime PM is enabled.

Also added pm_device_runtime_init_suspended to indicate that device is
in a suspended state (will prevent pm_device_runtime_enable to suspend
the device again).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>