dt-bindings: timer: Add T-HEAD TH1520 clint

Add compatible string for the T-HEAD TH1520 clint.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Documentation/devicetree/bindings/timer/sifive,clint.yaml

@@ -37,6 +37,7 @@ properties:
       - items:
           - enum:
               - allwinner,sun20i-d1-clint
+              - thead,th1520-clint
           - const: thead,c900-clint
       - items:
           - const: sifive,clint0