dt-bindings: nvmem: u-boot,env: add MAC's #nvmem-cell-cells

U-Boot's "ethaddr" environment variable is very often used to store
*base* MAC address. It's used as a base for calculating addresses for
multiple interfaces. It's done by adding proper values. Actual offsets
are picked by manufacturers and vary across devices.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230404172148.82422-34-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/nvmem/u-boot,env.yaml

@@ -50,7 +50,11 @@ properties:
 
   ethaddr:
     type: object
-    description: Ethernet interface's MAC address
+    description: Ethernet interfaces base MAC address.
+    properties:
+      "#nvmem-cell-cells":
+        description: The first argument is a MAC address offset.
+        const: 1
 
 additionalProperties: false
 
@@ -72,6 +76,7 @@ examples:
             reg = <0x40000 0x10000>;
 
             mac: ethaddr {
+                #nvmem-cell-cells = <1>;
             };
         };
     };