dt-bindings: net: Document fsl,erratum-a009885

Update FMan binding documentation with the newly added workaround for
erratum A-009885.

Signed-off-by: Tobias Waldekranz <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

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

···
 		The settings and programming routines for internal/external
 		MDIO are different. Must be included for internal MDIO.
 
+- fsl,erratum-a009885
+		Usage: optional
+		Value type: <boolean>
+		Definition: Indicates the presence of the A009885
+		erratum describing that the contents of MDIO_DATA may
+		become corrupt unless it is read within 16 MDC cycles
+		of MDIO_CFG[BSY] being cleared, when performing an
+		MDIO read operation.
+
 - fsl,erratum-a011043
 		Usage: optional
 		Value type: <boolean>