net: davinci_mdio: document missed "ti, am4372-mdio" compat string

Document missed "ti,am4372-mdio" compat string used for TI am437x SoC
(am4372.dtsi).

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>