mtd: bcm63xxpart: check version marker string for newer CFEs

Recent CFEs do not contain the CFE1CFE1 magic anymore, so check for the
"cfe-v" version marker string instead. As very old CFEs do not have
this string, leave the CFE1CFE1 magic as a fallback for detection.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>