msm: adsprpc: protect access of context table

Add spin lock for accessing the context table to avoid
use after free in case of complete signal. Maintain subsystem
specific context table instead of global context table for
optimizing search performance.

Change-Id: I4df3a905a142048972ff5cdda3fdd93ee4c527bb
Acked-by: Tadakamalla Krishnaiah <ktadakam@qti.qualcomm.com>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>
Signed-off-by: Himateja Reddy <hmreddy@codeaurora.org>