[dma/rtl] CHUNK_DATA_SIZE needs to be a multiple of TRANSFER_WIDTH

In DMA, we have three different sizes:
- TOTAL_DATA_SIZE: number of total bytes transferred.
- CHUNK_DATA_SIZE: a chunk is a slice of that transfer job.
- TRANSFER_WIDTH: these are the number of bytes per transaction.

This RTL correctly computes the remaining bytes even when the CHUNK_DATA_SIZE is not a multiple of the TRANSFER_WIDTH:
https://github.com/lowRISC/opentitan/blob/8fcd27c579b1793d21a1542d6370291cf17af146/hw/ip/dma/rtl/dma.sv#L1150-L1153

However, this RTL:
https://github.com/lowRISC/opentitan/blob/8fcd27c579b1793d21a1542d6370291cf17af146/hw/ip/dma/rtl/dma.sv#L957-L959
https://github.com/lowRISC/opentitan/blob/8fcd27c579b1793d21a1542d6370291cf17af146/hw/ip/dma/rtl/dma.sv#L763-L764
https://github.com/lowRISC/opentitan/blob/8fcd27c579b1793d21a1542d6370291cf17af146/hw/ip/dma/rtl/dma.sv#L776
always uses the TRANSFER_WIDTH size to advance. So a transaction where the CHUNK_DATA_SIZE is not a multiple of the TRANSFER_WIDTH, bytes can get lost.

In an example:
TOTAL_DATA_SIZE=12, CHUNK_DATA_SIZE=6, TRANSFER_WIDTH=4:
Chunk0, txn0: remaining_bytes=6, bytes written=4 (0...3), counter advances +4
Chunk0, txn1: remaining_bytes=2, bytes_written=2 (4...5), counter advances +4 <- wrong
Chunk1, txn0: remaining bytes=4, bytes_written=4 (8...11), counter advances +4 <- done
So bytes 6 and 7 are never written.

This was not detected in DV as DV constrained the chunk data and transwer width accordingly:
https://github.com/lowRISC/opentitan/blob/8fcd27c579b1793d21a1542d6370291cf17af146/hw/ip/dma/dv/env/dma_seq_item.sv#L340-L345

To fix this in RTL, the counting logic should be adapted.
To fix this in SW, make sure that CHUNK_DATA_SIZE is a multiple of TRANSFER_WIDTH.

Thanks to Vishal Bhogade (@vishalbb-git) for reporting this!

Start in hw/ip/dma/rtl/dma.sv at lines 763-764, 776, 957-959, and 1150-1153 to trace how counters advance for partial transactions. Then inspect hw/ip/dma/dv/env/dma_seq_item.sv at lines 340-345 and run or extend DMA DV with non-multiple CHUNK_DATA_SIZE and TRANSFER_WIDTH values. Done means the bytes in the example are all transferred, with the RTL or software constraint behavior made consistent.

Written by the indexing model from the issue text.