dt-bindings: timer: sifive,clint: add comaptibles for T-Head's C9xx

T-Head C906/C910 CLINT is not compliant to SiFive ones (and even not
compliant to the newcoming ACLINT spec) because of lack of mtime
register.

Add a compatible string formatted like the C9xx-specific PLIC
compatible, and do not allow a SiFive one as fallback because they're
not really compliant.

Signed-off-by: Icenowy Zheng <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Samuel Holland <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Daniel Lezcano <[e-mail]>

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

···
   property of "/cpus" DT node. The "timebase-frequency" DT property is
   described in Documentation/devicetree/bindings/riscv/cpus.yaml
 
+  T-Head C906/C910 CPU cores include an implementation of CLINT too, however
+  their implementation lacks a memory-mapped MTIME register, thus not
+  compatible with SiFive ones.
+
 properties:
   compatible:
     oneOf:
···
               - starfive,jh7100-clint
               - canaan,k210-clint
           - const: sifive,clint0
+      - items:
+          - enum:
+              - allwinner,sun20i-d1-clint
+          - const: thead,c900-clint
       - items:
           - const: sifive,clint0
           - const: riscv,clint0