ARM: vexpress/TC2: add cpu clock support

On TC2, the cpu clocks are controlled by the external M3 microcontroller
and SPC provides the interface between the CPU and the power controller.

The generic cpufreq drivers use the clock APIs to get the cpu clocks.
This patch add virtual spc clocks for all the cpus to control the cpu
operating frequency via the clock framework.

Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Pawel Moll <Pawel.Moll@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>