ARM: Kirkwood: Add missing DT reg property to cpu@0

The kirkwood.dtsi cpu@0 node is missing the mandatory reg property.
This causes of_get_cpu_node() to fail to find the node and as a result
the cpufreq driver fails in its probe function.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>