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>