dts: mt7629: enable JTAG pins by default

The EPHY LEDs belongs to the built-in FE switch of MT7629, which is barely
used. These LED pins on reference boards are used as JTAG socket. So it's
a good idea to change the default state to JTAG, and this will make it
convenience for debugging.

diff --git a/arch/arm/dts/mt7629-rfb.dts b/arch/arm/dts/mt7629-rfb.dts
index df43cc4..f2e4e95 100644
--- a/arch/arm/dts/mt7629-rfb.dts
+++ b/arch/arm/dts/mt7629-rfb.dts
@@ -36,6 +36,16 @@
 };
 
 &pinctrl {
+	state_default: pinmux_conf {
+		u-boot,dm-pre-reloc;
+
+		mux {
+			function = "jtag";
+			groups = "ephy_leds_jtag";
+			u-boot,dm-pre-reloc;
+		};
+	};
+
 	snfi_pins: snfi-pins {
 		mux {
 			function = "flash";