dt-bindings: net: xgmac_mdio: Add "clock-frequency" and "suppress-preamble"

The driver now supports the standard "clock-frequency" and
"suppress-preamble" properties, do document them in the binding
description.
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl-fman.txt

@@ -388,6 +388,25 @@ PROPERTIES
 		Value type: <prop-encoded-array>
 		Definition: A standard property.
 
+- clocks
+		Usage: optional
+		Value type: <phandle>
+		Definition: A reference to the input clock of the controller
+		from which the MDC frequency is derived.
+
+- clock-frequency
+		Usage: optional
+		Value type: <u32>
+		Definition: Specifies the external MDC frequency, in Hertz, to
+		be used. Requires that the input clock is specified in the
+		"clocks" property. See also: mdio.yaml.
+
+- suppress-preamble
+		Usage: optional
+		Value type: <boolean>
+		Definition: Disable generation of preamble bits. See also:
+		mdio.yaml.
+
 - interrupts
 		Usage: required for external MDIO
 		Value type: <prop-encoded-array>