ramips: improve support for H3C TX180x series devices

1. Explicitly declare gpio pin groups to ensure that gpio works properly.

2. Override bootargs in device tree to avoid modifying u-boot envs during
   initial installation.

Tested on H3C TX1801 Plus

Signed-off-by: Shiji Yang <yangshiji66@qq.com>

target/linux/ramips/dts/mt7621_h3c_tx180x.dtsi

@@ -16,6 +16,7 @@
 
 	chosen {
 		bootargs = "console=ttyS0,115200";
+		bootargs-override = "console=ttyS0,115200";
 	};
 
 	keys {
@@ -141,6 +142,13 @@
 	status = "disabled";
 };
 
+&state_default {
+	gpio {
+		groups = "jtag";
+		function = "gpio";
+	};
+};
+
 &switch0 {
 	ports {
 		port@1 {