dt-bindings: opp: Add msm8939 to the compatible list

msm8939 will uses this driver instead of the generic dt-cpufreq. Add to the
compatible list.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>