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>