arm64: defconfig: enable IOSCHED_DEADLINE

For certain workloads the deadline IO scheduler offers
particular advantages over other schedulers and has shown
to perform better, so enable it.

The default IO scheduler is unaffected by this change, and
currently is CFQ.

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>