mhi: core: serialize execution environment and power off changes

Execution environment and power off changes can be serialized
to avoid changing these important controller variables from
multiple places and serialize handling of all state transitions.

Change-Id: I394e5ff4992d7c911e3889f96057d8467d79509c
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>