drivers: soc: add mutex to prevent response being processed twice

Add a mutex to prevent two threads from processing the same response
at the same time. This ensures responses are processed completely and
sequentially.

CRs-Fixed: 1116015
Change-Id: Id2ef32edb939f8af2850b54bd6f6f447939c0732
Signed-off-by: Siena Richard <sienar@codeaurora.org>