dt-bindings: Add documentation for Tegra186 Denver

Update arm/cpus.txt with the compatible string for the Denver CPU found
on Tegra186 SoCs.
Signed-off-by: Thierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/arm/cpus.txt

@@ -178,6 +178,7 @@ nodes to be present and contain the properties described below.
 			    "marvell,pj4b"
 			    "marvell,sheeva-v5"
 			    "nvidia,tegra132-denver"
+			    "nvidia,tegra186-denver"
 			    "qcom,krait"
 			    "qcom,kryo"
 			    "qcom,scorpion"