dt-bindings: interrupt-controller: Add T-HEAD's TH1520 PLIC

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

Documentation/devicetree/bindings/interrupt-controller/sifive,plic-1.0.0.yaml

@@ -65,6 +65,7 @@ properties:
       - items:
           - enum:
               - allwinner,sun20i-d1-plic
+              - thead,th1520-plic
           - const: thead,c900-plic
       - items:
           - const: sifive,plic-1.0.0