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>

Documentation/devicetree/bindings/dma/nvidia,tegra210-adma.txt

@@ -4,7 +4,9 @@ The Tegra Audio DMA controller that is used for transferring data
 between system memory and the Audio Processing Engine (APE).
 
 Required properties:
-- compatible: Must be "nvidia,tegra210-adma".
+- compatible: Should contain one of the following:
+  - "nvidia,tegra210-adma": for Tegra210
+  - "nvidia,tegra186-adma": for Tegra186 and Tegra194
 - reg: Should contain DMA registers location and length. This should be
   a single entry that includes all of the per-channel registers in one
   contiguous bank.