dt-binding: mtd: nand: Document the cs-gpios property

To reach higher capacities, arrays of chips are now pretty common.
Unfortunately, most of the controllers have been designed a decade ago
and did not all anticipate the need for several chip-selects. The new
cs-gpios property allows to workaround this limitation by adding as many
GPIO chip-select as needed.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20210510171800.27225-1-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/mtd/nand-controller.yaml

@@ -38,6 +38,17 @@ properties:
 
   ranges: true
 
+  cs-gpios:
+    minItems: 1
+    maxItems: 8
+    description:
+      Array of chip-select available to the controller. The first
+      entries are a 1:1 mapping of the available chip-select on the
+      NAND controller (even if they are not used). As many additional
+      chip-select as needed may follow and should be phandles of GPIO
+      lines. 'reg' entries of the NAND chip subnodes become indexes of
+      this array when this property is present.
+
 patternProperties:
   "^nand@[a-f0-9]$":
     type: object
@@ -164,14 +175,19 @@ examples:
     nand-controller {
       #address-cells = <1>;
       #size-cells = <0>;
+      cs-gpios = <0>, <&gpioA 1>; /* A single native CS is available */
 
       /* controller specific properties */
 
       nand@0 {
-        reg = <0>;
+        reg = <0>; /* Native CS */
         nand-use-soft-ecc-engine;
         nand-ecc-algo = "bch";
 
         /* controller specific properties */
       };
+
+      nand@1 {
+        reg = <1>; /* GPIO CS */
+      };
     };