Documentation: DT: Add compatibility binding for Tegra186

Tegra186 Audio DMA controller has new updates from Tegra210 version.
Thus add new compatibility binding string and the same can be used
by Tegra194 as well.

Signed-off-by: Sameer Pujar <spujar@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>