tests: spi_loopback: Add configurations for a few nRF5 boards

This adds to the spi_loopback test board-specific configurations
for the following boards:
- nrf51_pca10028
- nrf52_pca10040
- nrf52840_pca10056

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>