mtd: nand: atmel: Document the new DT bindings

The old NAND bindings were not exactly describing the hardware topology
and were preventing definitions of several NAND chips under the same
NAND controller.

New bindings address these limitations and should be preferred over the
old ones for new SoCs/boards.
Old bindings are still supported for backward compatibility but are
marked deprecated in the doc.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>