mtd: docg3 trivial renaming

Change the name of the mtd so that it is simpler, and is easier to
cope with by mtdparts.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>