dt-bindings: net: ethernet-controller: fix typo in NVMEM

The correct property name is "nvmem-cell-names". This is what:
1. Was originally documented in the ethernet.txt
2. Is used in DTS files
3. Matches standard syntax for phandles
4. Linux net subsystem checks for

Fixes: 9d3de3c5 ("dt-bindings: net: Add YAML schemas for the generic Ethernet options")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ethernet-controller.yaml

@@ -49,7 +49,7 @@ properties:
     description:
       Reference to an nvmem node for the MAC address
 
-  nvmem-cells-names:
+  nvmem-cell-names:
     const: mac-address
 
   phy-connection-type: