ARM: dts: tango4: Make /serial compatible with ns16550a

ralink,rt2880-uart is compatible with ns16550a and all other
instances of RT2880 UART nodes include it in the compatible property.
Add it also here, to make the binding schema simpler.
Signed-off-by: NLubomir Rintel <lkundrak@v3.sk>
Reviewed-by: NRob Herring <robh@kernel.org>
Acked-by: NMans Rullgard <mans@mansr.com>
Link: https://lore.kernel.org/r/20200320174107.29406-8-lkundrak@v3.skSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm/boot/dts/tango4-common.dtsi

@@ -85,7 +85,7 @@
 		};
 
 		uart: serial@10700 {
-			compatible = "ralink,rt2880-uart";
+			compatible = "ralink,rt2880-uart", "ns16550a";
 			reg = <0x10700 0x30>;
 			interrupts = <1 IRQ_TYPE_LEVEL_HIGH>;
 			clock-frequency = <7372800>;