target/riscv/pmp: Restrict priviledged PMP to system-mode emulation

The RISC-V Physical Memory Protection is restricted to privileged
modes. Restrict its compilation to QEMU system builds.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>