spapr: fix possible memory leak

get_boot_devices_list() will malloc memory, spapr_finalize_fdt
doesn't free it.

Signed-off-by: Chenliang <chenliang88@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
