cpufreq: ap806: add cpufreq driver for Armada 8K

Add cpufreq driver for Marvell AP-806 found on Aramda 8K.
The AP-806 has DFS (Dynamic Frequency Scaling) with coupled
clock domain for two clusters, so this driver will directly
use generic cpufreq-dt driver as backend.

Based on the work of Omri Itach <omrii@marvell.com>.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>