tests: drivers: spi: spi_loopback: impl pm_device_runtime support

Implement pm_device_runtime support to spi_loopback test suite.

Specifically:
- call pm_device_runtime_get() on SPI bus before using SPI bus
- call pm_device_runtime_put() once SPI bus is no longer needed.
  This is after transceive has been performed, or after calling
  spi_release_dt() after a transceive has been performed.

This addition has no affect if PM_DEVICE_RUNTIME is not enabled
on a board/test case basis.

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