mtd: nand: document the NAND controller/NAND chip DT representation

Standardize the NAND controller/NAND chip DT representation. Now, all new
NAND controller drivers should comply with this representation, even if
they are only supporting a single NAND chip.

Existing drivers can keep support for the old representation (where only
the NAND chip was described), but are encouraged to also support the new
one.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>