ARM: dts: imx53: Add a label for the PMU node

Add a label for the PMU node so that the board dts may be able to
pass the 'secure-reg-access' property like this:

&pmu {
	secure-reg-access;
};

This also makes it consistent with the PMU node in imx6qdl.dtsi
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx53.dtsi

@@ -113,7 +113,7 @@ osc {
 		};
 	};
 
-	pmu {
+	pmu: pmu {
 		compatible = "arm,cortex-a8-pmu";
 		interrupt-parent = <&tzic>;
 		interrupts = <77>;