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>