dt-bindings: net: ftgmac100: describe phy-handle and MDIO

Add the phy-handle and MDIO description and add the example with
PHY and MDIO nodes.

Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>