cpufreq: Add DVFS support for Armada 37xx

This patch adds DVFS support for the Armada 37xx SoCs

There are up to four CPU frequency loads for Armada 37xx controlled by
the hardware.

This driver associates the CPU load level to a frequency, then the
hardware will switch while selecting a load level.

The hardware also can associate a voltage for each level (AVS support)
but it is not yet supported

Tested-by: Andre Heider <a.heider@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>