loongson: 2k1000: dts: fix uart default as 4 line mode

Changelog: 2k1000 uart0默认为4线模式
Signed-off-by: maoxiaochuan <maoxiaochuan@loongson.cn>

arch/loongarch/boot/dts/loongson2k1000.dtsi

@@ -464,6 +464,8 @@
 			clock-frequency = <125000000>;
 			interrupt-parent = <&liointc0>;
 			interrupts = <0x0 IRQ_TYPE_LEVEL_HIGH>;
+			pinctrl-names = "default";
+			pinctrl-0 = <&uart0_4_default>;
 			no-loopback-test;
 			status = "okay";
 		};