target/riscv: Fix mret exception cause when no pmp rule is configured

target/riscv: Fix mret exception cause when no pmp rule is configured

The priv spec v1.12 says:

  If no PMP entry matches an M-mode access, the access succeeds. If
  no PMP entry matches an S-mode or U-mode access, but at least one
  PMP entry is implemented, the access fails. Failed accesses generate
  an instruction, load, or store access-fault exception.

At present the exception cause is set to 'illegal instruction' but
should have been 'instruction access fault'.

Fixes: d102f19a2085 ("target/riscv/pmp: Raise exception if no PMP entry is configured")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221205065303.204095-1-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>