arm64: dts: rockchip: fix rock64 gmac2io stability issues

This commit enables thresh dma mode as this forces to disable checksuming,
and chooses delay values which make the interface stable.

These changes are needed, because ROCK64 is faced with two problems:
1. tx checksuming does not work with packets larger than 1498,
2. the default delays for tx/rx are not stable when using 1Gbps connection.

Delays were found out with:
https://github.com/ayufan-rock64/linux-build/tree/master/recipes/gmac-delays-testSigned-off-by: NKamil Trzciński <ayufan@ayufan.eu>
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>

arch/arm64/boot/dts/rockchip/rk3328-rock64.dts

@@ -132,17 +132,16 @@
 	assigned-clocks = <&cru SCLK_MAC2IO>, <&cru SCLK_MAC2IO_EXT>;
 	assigned-clock-parents = <&gmac_clkin>, <&gmac_clkin>;
 	clock_in_out = "input";
-	/* shows instability at 1GBit right now */
-	max-speed = <100>;
 	phy-supply = <&vcc_io>;
 	phy-mode = "rgmii";
 	pinctrl-names = "default";
 	pinctrl-0 = <&rgmiim1_pins>;
+	snps,force_thresh_dma_mode;
 	snps,reset-gpio = <&gpio1 RK_PC2 GPIO_ACTIVE_LOW>;
 	snps,reset-active-low;
 	snps,reset-delays-us = <0 10000 50000>;
-	tx_delay = <0x26>;
-	rx_delay = <0x11>;
+	tx_delay = <0x24>;
+	rx_delay = <0x18>;
 	status = "okay";
 };