dt-bindings: dmaengine: Add doc for tegra gpcdma

Add DT binding document for Nvidia Tegra GPCDMA controller.

Signed-off-by: Rajesh Gumasta <rgumasta@nvidia.com>
Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20220225132044.14478-2-akhilrajeev@nvidia.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>