mhi: core: Check for RDDM cookie set by device to indicate readiness

If the device is unable to send the mission mode execution
environment change event but has already entered mission mode
with the ability to allow ramdump collection, it can set a unique
cookie pattern to indicate the availability of ramdumps. Allow
the controller to query for this unique pattern upon any bootup
failure or timeout.

Change-Id: Id1f58b75d3f22e9e21b142cbea74d61cc65615b4
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>