From another Nordic DevZone question:

The Enable register is the key; there are 3 enable bits on the nRF52840 which are used 1 bit each for SPI enable, Rx DMA enable and Tx DMA enable. With the Enable register set to 0x0 all are disabled; 0x1 means SPI with no DMA; 0x7 SPIM with both Rx and Tx DMA. DMA-related registers are ignored if DMA enable bits are not set.