hw/xtensa/xtfpga:fix leak of fdevice tree blob

The device tree blob returned by load_device_tree is malloced.
We should free it after cpu_physical_memory_write().
Reported-by: NEuler Robot <euler.robot@huawei.com>
Signed-off-by: NChen Qun <kuhn.chenqun@huawei.com>
Acked-by: NMax Filippov <jcmvbkbc@gmail.com>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Message-Id: <20200218091154.21696-4-kuhn.chenqun@huawei.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

hw/xtensa/xtfpga.c

@@ -380,6 +380,7 @@ static void xtfpga_init(const XtfpgaBoardDesc *board, MachineState *machine)
             cur_tagptr = put_tag(cur_tagptr, BP_TAG_FDT,
                                  sizeof(dtb_addr), &dtb_addr);
             cur_lowmem = QEMU_ALIGN_UP(cur_lowmem + fdt_size, 4 * KiB);
+            g_free(fdt);
         }
 #else
         if (dtb_filename) {