drivers: gpio: smartbond: fix PM_DEVICE_DEFINE usage

The gpio_smartbond driver incorrectly uses the static
PM_DEVICE_DEFINE() and PM_DEVICE_GET() macros when creating a driver
instance from a devicetree instance number.

Update to use PM_DEVICE_DT_INST_DEFINE() and
PM_DEVICE_DT_INST_GET() macros.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>