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

Documentation/devicetree/bindings/net/davinci-mdio.txt

@@ -2,7 +2,9 @@ TI SoC Davinci/Keystone2 MDIO Controller Device Tree Bindings
 ---------------------------------------------------
 
 Required properties:
-- compatible		: Should be "ti,davinci_mdio" or "ti,keystone_mdio"
+- compatible		: Should be "ti,davinci_mdio"
+			  and "ti,keystone_mdio" for Keystone 2 SoCs
+			  and "ti,am4372-mdio" for am472x SoC
 - reg			: physical base address and size of the davinci mdio
 			  registers map
 - bus_freq		: Mdio Bus frequency