PM / OPP: Rename OPP nodes as opp@<opp-hz>

It would be better to name OPP nodes as opp@<opp-hz> as that will ensure
that multiple DT nodes don't contain the same frequency. Of course we
expect the writer to name the node with its opp-hz frequency and not any
other frequency.

And that will let the compile error out if multiple nodes are using the
same opp-hz frequency.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>