mtd: bcm63xxpart: don't assume NVRAM is always the fourth partition

Instead of referencing the sizes of fixed partitions, use the
precomputed CFE/NVRAM lengths.

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>