gpio: stm32: initialise according to `zephyr,pm-device-runtime-auto`

Don't automatically disable all GPIO ports just because
`PM_DEVICE_RUNTIME` is enabled. Require the user to explicitly call
`pm_device_runtime_enable` on the port, or add
`zephyr,pm-device-runtime-auto` to the devicetree node.

Signed-off-by: Jordan Yates <jordan@embeint.com>