drivers: dma: siwx91x: distinguishing mem to mem transfers

Introduced a new variable in the `dma_siwx91x_channel_info`
structure to provide a clean way to differentiate transfer
directions. This enhancement is utilized to trigger software
requests specifically for memory-to-memory transfers

Signed-off-by: Sai Santhosh Malae <Santhosh.Malae@silabs.com>