arm64: dts: uniphier: stabilize ethernet of LD20 reference board

Currently, the ethernet RGMII mode on the LD20 reference board is
unstable.

The default drive-strength of ethernet TX pins is too strong because
there is no dumping resistor on the TX lines on the board.

Weaken the drive-strength to make the ethernet more stable.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/arm64/boot/dts/socionext/uniphier-ld20-ref.dts

@@ -67,3 +67,11 @@ ethphy: ethphy@0 {
 		reg = <0>;
 	};
 };
+
+&pinctrl_ether_rgmii {
+	tx {
+		pins = "RGMII_TXCLK", "RGMII_TXD0", "RGMII_TXD1",
+		       "RGMII_TXD2", "RGMII_TXD3", "RGMII_TXCTL";
+		drive-strength = <9>;
+	};
+};