mhi: core: add wait for m2 state change before suspend

Some controllers would like to wait for MHI state to change
to M2 before continuing with suspend. Allow for that by
providing a configurable timeout in milliseconds.

Change-Id: I9b700074de427dfc9f065e795e4fa2db26daae63
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>