ramips: fix mt7628 eval board pinmux

There is no pinmux group "jtag" for mt7628 and the pinmux driver fails
to load due to the use of the not existing group.

Fixes: FS#1515

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/dts/MT7628.dts

@@ -15,7 +15,7 @@
 &pinctrl {
 	state_default: pinctrl0 {
 		gpio {
-			ralink,group = "i2c", "jtag";
+			ralink,group = "i2c";
 			ralink,function = "gpio";
 		};
 	};