PM / OPP: Remove 'operating-points-names' binding

These aren't used until now by any DT files and wouldn't be used now as
we have a better scheme in place now, i.e. opp-property-<name>
properties.

Remove the (useless) binding without breaking ABI.

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>