esoc: Add provision to handle shutdown request in userspace

In certain scenarios, modem shutdown requests are handled in
userspace. Enhance request engine of esoc driver to send
shutdown requests to userspace.

Also, during a shutdown, avoid setting status to 0, if line is
not a power source. There can be multiple mdms monitoring status
line. This can otherwise be misinterpreted as an unexpected reset
by other mdms.

Change-Id: I9c20a86e76f892cc61dbfb814202b26e5cce3e96
Signed-off-by: Arun KS <arunks@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>