dt-bindings: serial: 8250: Add Loongson 3A4000 uart compatible

The UART controller on Loongson 3A4000 is compatible with Loongson
2K1500, which is NS16550A-compatible with an additional fractional
frequency divisor register.

Add loongson,ls3a4000-uart as compatible with loongson,ls2k1500-uart.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Rong Zhang <rongrong@oss.cipunited.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://patch.msgid.link/20260315184301.412844-2-rongrong@oss.cipunited.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Rongronggg9

Documentation/devicetree/bindings/serial/8250.yaml

@@ -179,6 +179,7 @@ properties:
           - const: ns16550a
       - items:
           - enum:
+              - loongson,ls3a4000-uart
               - loongson,ls3a5000-uart
               - loongson,ls3a6000-uart
               - loongson,ls2k2000-uart