boards/arm/mps2_an521: Add missing timer labels to dts

The bindings for arm,cmsdk-{d}timer requires a label so add it into the
dts since its missing on mps2_an521.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>