msm: mdm: add support for logging the mdm SSR failure reason

If the external modem is restarted because of a fatal error,
it may have saved a string that suggests the reason for the
error. Retrieve that string and log it as a kernel message.

Change-Id: I4fdfbabd464414564c795b53755fe97382da00d5
Signed-off-by: Joel King <joelking@codeaurora.org>