mips: dts: ralink: mt7621: associate uart1_pins with serial0

Add missing pinctrl-name and pinctrl-0 properties to declare
that the uart1_pins group is associated with serial0.
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/ralink/mt7621.dtsi

@@ -123,6 +123,9 @@ serial0: serial@c00 {
 			reg-shift = <2>;
 			reg-io-width = <4>;
 			no-loopback-test;
+
+			pinctrl-names = "default";
+			pinctrl-0 = <&uart1_pins>;
 		};
 
 		spi0: spi@b00 {