arm64: defconfig: Enable MEGARAID_SAS

Our new dev board (Huawei D06CS) has a MegaRAID SAS HBA as the storage
controller, and we cannot access the HiSilicon SAS HBA, which we normally
use; so enable the driver config option so we can boot a distro with the
default defconfig.

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