target/microblaze: Add the opcode-0x0-illegal CPU property

Add the opcode-0x0-illegal CPU property to control if the core
should trap opcode zero as illegal.

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>

target/microblaze/cpu.h

@@ -303,6 +303,7 @@ struct MicroBlazeCPU {
         bool endi;
         bool dopb_bus_exception;
         bool iopb_bus_exception;
+        bool opcode_0_illegal;
         char *version;
         uint8_t pvr;
     } cfg;