clk: qcom: Fix race condition when unvoting regulators in sync_state

Normally clk_vote_vdd_level() and clk_unvote_vdd_level() are called
while holding the clock framework's prepare_lock since they are called
in the prepare paths. However, we're also calling clk_unvote_vdd_level()
from qcom_cc_sync_state() when removing proxy votes. This happens
outside of the clock framework, so the prepare_lock isn't protecting
this case. If the sync_state callback fires while another thread is
voting/unvoting voltage from a normal clock call, then there's a race
condition that can result in regulators being set to the incorrect
voltage or being pre-maturely disabled.

Add a mutex to protect this case.

Change-Id: Ib979e2edaaa5824dc80f7d4f065d72ca0b1a4444
Signed-off-by: Mike Tipton <mdtipton@codeaurora.org>