net/fsl_pq_mdio: Document supported compatibles

The device tree binding(s) document has fallen out of sync with the
driver code. Update the list of supported devices to reflect current
driver capabilities

Change-Id: I440d8de2ee2d9c3b7b23e69b3da851cab18a4c9a
Signed-off-by: NShruti Kanetkar <Kanetkar.Shruti@gmail.com>
Signed-off-by: NEmil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl-tsec-phy.txt

@@ -8,7 +8,16 @@ of how to define a PHY.
 Required properties:
   - reg : Offset and length of the register set for the device
   - compatible : Should define the compatible device type for the
-    mdio.  Currently, this is most likely to be "fsl,gianfar-mdio"
+    mdio. Currently supported strings/devices are:
+	- "fsl,gianfar-tbi"
+	- "fsl,gianfar-mdio"
+	- "fsl,etsec2-tbi"
+	- "fsl,etsec2-mdio"
+	- "fsl,ucc-mdio"
+	- "fsl,fman-mdio"
+    When device_type is "mdio", the following strings are also considered:
+	- "gianfar"
+	- "ucc_geth_phy"
 
 Example: