target/microblaze: Add the ill-opcode-exception property

target/microblaze: Add the ill-opcode-exception property

Add the ill-opcode-exception property to control if illegal
instructions will raise exceptions.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>