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

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>