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>