qcom: clock-generic: Optionally set a safe freq on the safe parent

Some mux implementations may have safe parents that are
muxes themselves, and only one of the mux inputs is
actually considered "safe". To allow the selection of
this "safe" input, set a frequency on the safe parent
before switching to it.

Change-Id: I7007eb3ffd444ba38696d36564fef9c04a624da2
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>