ARM: dts: i.MX53: add pmu node

The i.MX53 has a Cortex-A8 Performance Monitor Unit.

Add it to the dtsi.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>