mtd: docg3 refactor cascade floors structure

Group floors into a common cascade structure. This will provide a common
structure to store common data to all cascaded docg3 chips, like IO
addressing, locking protection.

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>