doc: dt: atmel_nand: Reword the documentation

Do not mention which chips supporting the PMECC controller, as it a
duplicate of the information in the chips' device trees.

Use common terms when describing the sub-node for the NAND Flash
controller.

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