DMA Configuration register
| TX_THD_VAL | TX FIFO Level DMA Trigger If the TX FIFO level is less than this value, then the TX FIFO DMA interface will send a signal to system DMA to notify that TX FIFO is ready to receive data from memory. |
| TX_EN | TX DMA channel enable |
| RX_THD_VAL | Rx FIFO Level DMA Trigger If the RX FIFO level is greater than this value, then the RX FIFO DMA interface will send a signal to the system DMA to notify that RX FIFO has characters to transfer to memory. |
| RX_EN | RX DMA channel enable |