target/riscv: Rename riscv_cpu_mmu_index to riscv_env_mmu_index

target/riscv: Rename riscv_cpu_mmu_index to riscv_env_mmu_index

Free up the riscv_cpu_mmu_index name for other usage;
emphasize that the argument is 'env'.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>