mtd: nand: change "AMD" manuf. ID to "AMD/Spansion"

This manufacturer ID is used under the name Spansion.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>