ARM: dts: uniphier: add CPU clocks and OPP table for PXs2 SoC

Add a CPU clock to every CPU node and a CPU OPP table to use the
generic cpufreq driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>