tests: drivers: spi: loopback: test device_deinit

Introduce test for device_deinit() which deinitializes the spi
bus, then configures miso as input, mosi as output using gpio,
utilizing the loopback to test directly controlling the pins.
Then reinit the spi device.

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