ARM: dts: sun7i: pcduino3-nano: enable RGMII RX/TX delay on PHY

The RX/TX delays for the Ethernet PHY on the Linksprite pcDuino 3 Nano
are configured in hardware, using resistors that are populated to pull
the RTL8211E's RXDLY/TXDLY pins low or high as needed.

phy-mode should be set to rgmii-id to reflect this. Previously it was
set to rgmii, which used to work but now results in the delays being
disabled again as a result of the bugfix in commit bbc4d71d ("net:
phy: realtek: fix rtl8211e rx/tx delay config").

Tested on two pcDuino 3 Nano boards purchased in 2015. Without this fix,
Ethernet works unreliably on one board and doesn't work at all on the
other.

Fixes: 061035d4 ("ARM: dts: sun7i: Add dts file for pcDuino 3 Nano board")
Signed-off-by: Adam Sampson <ats@offog.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20201123174739.6809-1-ats@offog.org

arch/arm/boot/dts/sun7i-a20-pcduino3-nano.dts

 /*
- * Copyright 2015 Adam Sampson <ats@offog.org>
+ * Copyright 2015-2020 Adam Sampson <ats@offog.org>
  *
  * This file is dual-licensed: you can use it either under the terms
  * of the GPL or the X11 license, at your option. Note that this dual
@@ -115,7 +115,7 @@ &gmac {
 	pinctrl-names = "default";
 	pinctrl-0 = <&gmac_rgmii_pins>;
 	phy-handle = <&phy1>;
-	phy-mode = "rgmii";
+	phy-mode = "rgmii-id";
 	status = "okay";
 };