cpufreq: dt: add Socionext UniPhier SoCs support

Add compatible strings for Pro5, PXs2, LD6b, LD11, LD20 SoCs to use
the generic cpufreq driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>