target/riscv/pmp: fix no pmp illegal intrs

target/riscv/pmp: fix no pmp illegal intrs

As per the privilege specification, any access from S/U mode should fail
if no pmp region is configured and pmp is present, othwerwise access
should succeed.

Fixes: d102f19a208 (target/riscv/pmp: Raise exception if no PMP entry is configured)
Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211214092659.15709-1-nikita.shubin@maquefel.me
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>