mhi: core: add a power down status to notify controller

MHI controller can use power down status to take desired action
when core layer wants to issue a wake up in cases where it is
powering down.

Change-Id: I55c30b2a8bd703dcd6c994220b64023617540ed0
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>