dt-bindings: dma: ti: Add document for K3 PKTDMA

New binding document for
Texas Instruments K3 Packet DMA (PKTDMA).

PKTDMA is introduced as part of AM64.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201208090440.31792-13-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>