microblaze: Fix _reset() function

There is a need to disable VM before jump to zero reset vector.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Asserhall <stefan.asserhall@xilinx.com>

arch/microblaze/kernel/entry.S

@@ -958,6 +958,7 @@ ENTRY(_switch_to)
 	nop
 
 ENTRY(_reset)
+	VM_OFF
 	brai	0; /* Jump to reset vector */
 
 	/* These are compiled and loaded into high memory, then