arm64: tegra: Enable CPU sleep on Jetson Nano

Jetson Nano implements CPU sleep via PSCI, much like any of the other
Tegra X1 platforms. Enable the sleep states to allow the CPU to go into
lower power states when idle.
Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arm64/boot/dts/nvidia/tegra210-p3450-0000.dts

@@ -540,6 +540,12 @@ cpu@2 {
 		cpu@3 {
 			enable-method = "psci";
 		};
+
+		idle-states {
+			cpu-sleep {
+				status = "okay";
+			};
+		};
 	};
 
 	gpio-keys {