target/microblaze: Add the unaligned-exceptions property

Add the unaligned-exceptions property to control if the core
traps unaligned memory accesses.

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

@@ -306,6 +306,7 @@ struct MicroBlazeCPU {
         bool illegal_opcode_exception;
         bool opcode_0_illegal;
         bool div_zero_exception;
+        bool unaligned_exceptions;
         char *version;
         uint8_t pvr;
     } cfg;