ARM: mvebu: update Armada XP DT for dynamic frequency scaling

ARM: mvebu: update Armada XP DT for dynamic frequency scaling

In order to support dynamic frequency scaling:

 * the cpuclk Device Tree node needs to be updated to describe a
   second set of registers describing the PMU DFS registers.

 * the clock-latency property of the CPUs must be filled, otherwise
   the ondemand and conservative cpufreq governors refuse to work. The
   latency is high because the cost of a frequency transition is quite
   high on those CPUs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1404920715-19834-5-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>