arm64: dts: uniphier: add CPU clocks and OPP tables for LD20 SoC

Add a CPU clock to every CPU node and CPU OPP tables to use the
generic cpufreq driver.  All the CPUs in each cluster share the
same OPP table.

Note:
clock-latency-ns (300ns) was calculated based on the CPU-gear switch
sequencer spec; it takes 12 clock cycles on the sequencer running
at 50 MHz, plus a bit additional latency.

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