msm: clock: Make lock for vdd_class into mutex

There is no reason to hold a spinlock here anymore when the
vdd_class is only updated in non-atomic context. Move to using a
mutex instead. We couldn't do this before because voltage voting
was done in atomic context.

Change-Id: Ic0a4fa8f4255f8454d0b34d148cc5365eca8f139
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>