dt-bindings: net: add ethernet controller and phy sfp property

Document the missing sfp property for ethernet controllers (which
has existed for some time) which is being extended to ethernet PHYs.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>