cpufreq-dt: Add L2 frequency scaling support

Krait CPUs have a unified L2 cache (outside the cores, but on-die).
In addition to each CPU having its own clock which can be scaled
independently, the L2 has its own separate clock.

Allow scaling of the L2 clock, based on the current frequency of
each CPU core. Keep the L2 clock as close as possible to the maximum
frequency that the CPUs are currently running.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>