mips: dts: realtek: use "serial" instead of "uart" in node name

Update the node name for the UARTs to resolve the following dtbs_check
complaints:

  uart@2000: $nodename:0: 'uart@2000' does not match '^serial(@.*)?$'
  uart@2100: $nodename:0: 'uart@2100' does not match '^serial(@.*)?$'
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/realtek/rtl83xx.dtsi

@@ -22,7 +22,7 @@ soc: soc {
 		#size-cells = <1>;
 		ranges = <0x0 0x18000000 0x10000>;
 
-		uart0: uart@2000 {
+		uart0: serial@2000 {
 			compatible = "ns16550a";
 			reg = <0x2000 0x100>;
 
@@ -39,7 +39,7 @@ uart0: uart@2000 {
 			status = "disabled";
 		};
 
-		uart1: uart@2100 {
+		uart1: serial@2100 {
 			compatible = "ns16550a";
 			reg = <0x2100 0x100>;