clk: Vexpress-spc: Add clock driver

clk: Vexpress-spc: Add clock driver

This patch adds spc clock controller. In Vexpress cpu cluster clock is controlled via
spc controller and so it must be present in clk framework.

vexpress_clk_[of_]register_spc() registers cluster clocks with and without DT.
These are added as root clocks without any parents and their names are
"cluster[0|1|..]".

Now, platform must add clocks of all the cpus below these clusters. cpufreq
driver would get cpu clock and will do clk_get[set]_rate() on cpu clock, which
will then pass it to cluster clocks. And finally spc will get programmed.

This patch doesn't add non-DT clocks for clusters and cpus as i don't see a user
of that for now.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>