dt-bindings: mtd: atmel-nand: Document the nfc-io bindings

SAMA5 SoCs embed an advanced NAND controller logic to automate READ/WRITE
page accesses. This advanced logic is exposed through a separate I/O mem
range and is thus represented in a different node with its own compatible.

Document the bindings of this nfc-io block.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>