msm: clock-8960: Clean up handling of SoC differences in VDD setting fns

Clean up the code to make better use of the set_vdd ops to handle the
differences in the power rails between the various SoCs supported by this
driver. Instead of checking the SoC type for every call or having the code
for all SoCs inside a single function, split out the code into SoC specific
functions and set up the set_vdd ops during init.

Change-Id: I31ece44112847264a993d2df2773f47189b484a3
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>