[C31][DTS] fix ethernet delays

The current ethernet configuration sets delays in both MAC and PHY. This requires
to alter the drivers to allow forcing both delays, which is unnecessary.

This commit sets mode to rgmii-id and sets internal delays to tx=2.75ns and
rx=1.75ns. How these values were calculated: the RK3399 gmac is capable of a ~5ns delay.
The rx_delay and tx_delay properties can have values 0-100, so one step is ~0.05ns.

The new values have been tested using iperf3 (0 retr in both rx and tx).

arch/arm64/boot/dts/seco/seco-rk3399-c31.dtsi

@@ -855,7 +855,7 @@ pcal6416: gpio@20 {
 
 &gmac {
 	phy-supply = <&vcc_phy>;
-	phy-mode = "rgmii";
+	phy-mode = "rgmii-id";
 	phy-handle = <&eth_phy>;
 	clock_in_out = "output";
 	snps,reset-gpio = <&gpio3 RK_PB7 GPIO_ACTIVE_LOW>;
@@ -866,12 +866,10 @@ &gmac {
 	assigned-clock-rates = <125000000>;
 	pinctrl-names = "default";
 	pinctrl-0 = <&rgmii_pins &phy_rst>;
-	tx_delay = <0x28>;
-	rx_delay = <0x11>;
 	/* keep these phy properties below for u-boot
 	 * as DP83867 driver can't get them from phy node */
-	ti,rx-internal-delay = <DP83867_RGMIIDCTL_1_NS>;
-	ti,tx-internal-delay = <DP83867_RGMIIDCTL_1_NS>;
+	ti,rx-internal-delay = <DP83867_RGMIIDCTL_1_75_NS>;
+	ti,tx-internal-delay = <DP83867_RGMIIDCTL_2_75_NS>;
 	ti,fifo-depth = <DP83867_PHYCR_FIFO_DEPTH_8_B_NIB>;
 	status = "okay";
 
@@ -882,8 +880,8 @@ mdio {
 
 		eth_phy: eth-phy@9 {
 			reg = <9>;
-			ti,rx-internal-delay = <DP83867_RGMIIDCTL_1_NS>;
-			ti,tx-internal-delay = <DP83867_RGMIIDCTL_1_NS>;
+			ti,rx-internal-delay = <DP83867_RGMIIDCTL_1_75_NS>;
+			ti,tx-internal-delay = <DP83867_RGMIIDCTL_2_75_NS>;
 			ti,fifo-depth = <DP83867_PHYCR_FIFO_DEPTH_8_B_NIB>;
 		};
 	};