cpufreq: airoha: Add EN7581 CPUFreq SMCCC driver

Add simple CPU Freq driver for Airoha EN7581 SoC that control CPU
frequency scaling with SMC APIs and register a generic "cpufreq-dt"
device.

All CPU share the same frequency and can't be controlled independently.
CPU frequency is controlled by the attached PM domain.

Add SoC compatible to cpufreq-dt-plat block list as a dedicated cpufreq
driver is needed with OPP v2 nodes declared in DTS.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>