regulator: mem-acc-regulator: add locking for acc register writes

Some targets share a single ACC control register between two
independent application processor clusters.  Add locking around
ACC register updating calls in order to guarantee mutual
exclusion between the clusters on these targets.

Change-Id: If3bfdd6b6e798fafed4df9131b5620172cacd72f
Signed-off-by: David Collins <collinsd@codeaurora.org>