dt-bindings: serial: 8250: Add SpacemiT K1 uart compatible

Found SpacemiT's K1 uart controller is compatible with
Intel's Xscale uart, but it's still worth to introduce a new compatible.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Yixun Lan <dlan@gentoo.org>

Documentation/devicetree/bindings/serial/8250.yaml

@@ -111,7 +111,9 @@ properties:
               - mediatek,mt7623-btif
           - const: mediatek,mtk-btif
       - items:
-          - const: mrvl,mmp-uart
+          - enum:
+              - mrvl,mmp-uart
+              - spacemit,k1-uart
           - const: intel,xscale-uart
       - items:
           - enum: