target/riscv: Restrict SoftMMU mmu_index() to TCG

target/riscv: Restrict SoftMMU mmu_index() to TCG

Move riscv_cpu_mmu_index() to the TCG-specific file,
convert CPUClass::mmu_index() to TCGCPUOps::mmu_index().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250401080938.32278-17-philmd@linaro.org>