netlink: specs: add phy-index as a header parameter

Update the spec to take the newly introduced phy-index as a generic
request parameter.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/netlink/specs/ethtool.yaml

@@ -30,6 +30,9 @@ attribute-sets:
       -
         name: flags
         type: u32
+      -
+        name: phy-index
+        type: u32
 
   -
     name: bitset-bit