clk: Export the clock driver's voltage voting APIs

There could be special voltage requirements for a clock in addition
to what the clock framework already takes care of.
Export the APIs to vote on the corresponding voltage rails on behalf
on these clocks for use from the individual clock-type drivers.

Change-Id: I818d6f17176372791b6040bc8c5d6f7c40f697df
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>