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.

Cc: ilia.lin@kernel.org
Cc: robh+dt@kernel.org
Cc: krzk+dt@kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>