mhi: cntrl: qcom: Add fallback image support for debug mode

While in debug mode, in case the requested image is not found on the
file system, attempt to use the fallback image once before bailing out
on the power up.

Change-Id: I864adfc82f7a18dd186b0183148e5cbc9e24e7c5
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>