esoc: Add support for requesting shutdown

Allow external soc to issue a shutdown request to the primary
soc. Primary soc can perform cleanup of interface to external
soc if required.

Change-Id: I09bf47a8610274f3f7c5b5851aaf9f8e78f7b2cc
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>