dt-bindings: power: Add missing rpmpd smd performance level

The RPM_SMD_LEVEL_TURBO_HIGH is used by MSM8956/8976 and APQ variants:
add the definition.

Signed-off-by: Angelo G. Del Regno <kholk11@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>