Trace and Debug registers
| SOURCE_PIXEL | source pixels fetched from dma |
| LAST_BLOCK | indicate the encoding process for the last mcu of the picture |
| LAST_MCU | indicate the encoding process for the last block of the picture |
| LAST_DC | indicate the encoding process is at the header of the last block of the picture |
| PACKFIFO_READY | the jpeg pack_fifo ready signal, high active |