net/fsl: xgmac_mdio: Support preamble suppression

Support the standard "suppress-preamble" attribute to disable preamble
generation.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>