adsprpc: Handle early wake up signal support

Add spin lock for accessing the context table to avoid
use after free in case of complete signal. Context table
changed to channel wise for minimizing performance.

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