dt-bindings: net: ipq4019-mdio: document now supported clock-frequency

Document support for clock-frequency and add details on why this
property is needed and what values are supported.

From internal documentation, while other values are supported, the
correct function of the MDIO bus is not assured hence add only the
suggested supported values to the property enum.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/qcom,ipq4019-mdio.yaml

@@ -44,6 +44,21 @@ properties:
     items:
       - const: gcc_mdio_ahb_clk
 
+  clock-frequency:
+    description:
+      The MDIO bus clock that must be output by the MDIO bus hardware, if
+      absent, the default hardware values are used.
+
+      MDC rate is feed by an external clock (fixed 100MHz) and is divider
+      internally. The default divider is /256 resulting in the default rate
+      applied of 390KHz.
+
+      To follow 802.3 standard that instruct up to 2.5MHz by default, if
+      this property is not declared and the divider is set to /256, by
+      default 1.5625Mhz is select.
+    enum: [ 390625, 781250, 1562500, 3125000, 6250000, 12500000 ]
+    default: 1562500
+
 required:
   - compatible
   - reg