dt-bindings: add Qualcomm IPQ4019 MDIO bindings

This patch adds the binding document for the IPQ40xx MDIO driver.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Signed-off-by: David S. Miller <davem@davemloft.net>