As per j721s2 TRM the 48 bit address space is accessible by Udma. However in the case of MCSPI the driver method: MCSPI_dmaTransfer() takes as a second argument a pointer to SPI_Transaction*

where the Rx and Tx buffer pointers are defined as:

void txBuf; /!< void * to a buffer with data to be transmitted */
void rxBuf; /!< void * to a buffer to receive data */

 It is obvious that void* is not able to reach outside of 32 bit address boundary. And this limits the MCSPI DMA to the 32 bit address space.