msm: ADSPRPC: Fix deadlock during SSR

Deadlock is happening as mutex is locked before rpmsg_send and
SSR notifier thread is waiting on same mutex. rpmsg_send is
blocking call and returns only after notifying DSP. rpmsg is
not able to notify as notifier thread is blocked. Used separate
lock for guarding rpmsg device.

Change-Id: Idc8d85c6283c884fd99881c581e200ca48586d6b
Acked-by: Himateja Reddy <hmreddy@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>