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>

Documentation/devicetree/bindings/mtd/atmel-nand.txt

@@ -59,8 +59,22 @@ Required properties:
 - reg: should contain 2 register ranges. The first one is pointing to the PMECC
        block, and the second one to the PMECC_ERRLOC block.
 
+* SAMA5 NFC I/O bindings:
+
+SAMA5 SoCs embed an advanced NAND controller logic to automate READ/WRITE page
+operations. This interface to this logic is placed in a separate I/O range and
+should thus have its own DT node.
+
+- compatible: should be "atmel,sama5d3-nfc-io", "syscon".
+- reg: should contain the I/O range used to interact with the NFC logic.
+
 Example:
 
+	nfc_io: nfc-io@70000000 {
+		compatible = "atmel,sama5d3-nfc-io", "syscon";
+		reg = <0x70000000 0x8000000>;
+	};
+
 	pmecc: ecc-engine@ffffc070 {
 		compatible = "atmel,at91sam9g45-pmecc";
                 reg = <0xffffc070 0x490>,