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>
(cherry picked from commit af87a39a5f7cf6ef252b1aec3e2e6508a40e51f1)
Signed-off-by: Alex Shi <alex.shi@linaro.org>