dt-bindings: riscv: Add SiFive S7 compatible

Add a new compatible string in cpu.yaml for SiFive S7 CPU
core which is used on SiFive U74-MC core complex etc.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Hal Feng <hal.feng@starfivetech.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Author: hal-feng

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -35,6 +35,7 @@ properties:
               - sifive,e7
               - sifive,e71
               - sifive,rocket0
+              - sifive,s7
               - sifive,u5
               - sifive,u54
               - sifive,u7