LoongArch: dts: Add uart new compatible string

Add loongson,ls2k*-uart compatible string on uarts.

Co-developed-by: Haowei Zheng <[e-mail]>
Signed-off-by: Haowei Zheng <[e-mail]>
Signed-off-by: Binbin Zhou <[e-mail]>
Reviewed-by: Huacai Chen <[e-mail]>
Link: https://patch.msgid.link/8e[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

arch/loongarch/boot/dts/loongson-2k0500.dtsi

···
 		};
 
 		uart0: serial@1ff40800 {
-			compatible = "ns16550a";
+			compatible = "loongson,ls2k0500-uart", "ns16550a";
 			reg = <0x0 0x1ff40800 0x0 0x10>;
 			clock-frequency = <100000000>;
 			interrupt-parent = <&eiointc>;

arch/loongarch/boot/dts/loongson-2k1000.dtsi

···
 		};
 
 		uart0: serial@1fe20000 {
-			compatible = "ns16550a";
+			compatible = "loongson,ls2k1000-uart", "loongson,ls2k0500-uart", "ns16550a";
 			reg = <0x0 0x1fe20000 0x0 0x10>;
 			clock-frequency = <125000000>;
 			interrupt-parent = <&liointc0>;

arch/loongarch/boot/dts/loongson-2k2000.dtsi

···
 		};
 
 		uart0: serial@1fe001e0 {
-			compatible = "ns16550a";
+			compatible = "loongson,ls2k2000-uart", "loongson,ls2k1500-uart", "ns16550a";
 			reg = <0x0 0x1fe001e0 0x0 0x10>;
 			clock-frequency = <100000000>;
 			interrupt-parent = <&liointc>;