MIPS: ath79: ar9331: rename uart to serial node

schema violation was detected by the dtbs_check
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/qca/ar9331.dtsi

@@ -59,7 +59,7 @@ ddr_ctrl: memory-controller@18000000 {
 				#qca,ddr-wb-channel-cells = <1>;
 			};
 
-			uart: uart@18020000 {
+			uart: serial@18020000 {
 				compatible = "qca,ar9330-uart";
 				reg = <0x18020000 0x14>;