flash: nrf_qspi_nor: don't auto call `pm_device_runtime_enable`

Device drivers should not be calling `pm_device_runtime_enable` on
themselves, it should be left up to the application. If automatic
enablement is desired, `zephyr,pm-device-runtime-auto` exists as a
devicetree property.

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