msm: clock-local2: Add debug mux measurement support

In the near future, the debug muxes present in MSM clock
controllers will be modelled using the clock-generic
library. Add private data and a get_rate function to
support measurement using that library.

Change-Id: If8b1caac029190976fb98aa1f71267fc5ecfeb13
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>