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

The Loongson family have a mostly NS16550A-compatible UART and
High-Speed UART hardware with the exception of custom frequency divider
latch settings register.

Co-developed-by: Haowei Zheng <[e-mail]>
Signed-off-by: Haowei Zheng <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Binbin Zhou <[e-mail]>
Reviewed-by: Huacai Chen <[e-mail]>
Link: https://patch.msgid.link/2d[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/8250.yaml

···
               - nxp,lpc1850-uart
               - opencores,uart16550-rtlsvn105
               - ti,da830-uart
+              - loongson,ls2k0500-uart
+              - loongson,ls2k1500-uart
           - const: ns16550a
       - items:
           - enum:
···
               - nvidia,tegra194-uart
               - nvidia,tegra234-uart
           - const: nvidia,tegra20-uart
+      - items:
+          - enum:
+              - loongson,ls2k1000-uart
+          - const: loongson,ls2k0500-uart
+          - const: ns16550a
+      - items:
+          - enum:
+              - loongson,ls3a5000-uart
+              - loongson,ls3a6000-uart
+              - loongson,ls2k2000-uart
+          - const: loongson,ls2k1500-uart
+          - const: ns16550a
 
   reg:
     maxItems: 1