mhi: core: provide an API to retrieve device failure reason

When device asserts, the subsystem failure reason from device
can be read via an external API using the name of the MHI
controller.

Change-Id: Ic0bcba77e0a7e4c4a7570df3eda21e1f38b80277
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>