arm64: dts: zii-ultra: update MDIO speed and preamble

Update MDIO configuration with zii-ultra device to fully utilize
MDIO endpoint capabilities.  Device supports 12.5MHz clock and
doesn't require MDIO preamble.
Signed-off-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mq-zii-ultra.dtsi

@@ -131,6 +131,8 @@ &fec1 {
 	mdio {
 		#address-cells = <1>;
 		#size-cells = <0>;
+		clock-frequency = <12500000>;
+		suppress-preamble;
 		status = "okay";
 
 		switch: switch@0 {