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 <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>