target-microblaze: Correct the PVR array size

Correct the PVR array size, there are 13 PVR registers.

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

target/microblaze/cpu.h

@@ -277,7 +277,7 @@ struct CPUMBState {
     /* These fields are preserved on reset.  */
 
     struct {
-        uint32_t regs[16];
+        uint32_t regs[13];
     } pvr;
 };