dt-bindings: dma: altera-msgdma: make response port optional

Response port is not required in some configuration of the IP core.

Signed-off-by: Olivier Dautricourt <olivier.dautricourt@orolia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Link: https://lore.kernel.org/r/fb28146a23a182be9e5435c1d3e5cac36b372294.1623898678.git.olivier.dautricourt@orolia.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>