arm64: include VIRTIO_{MMIO,BLK} in defconfig

Currently, development on arm64 is aided by a Foundation_v8 emulator
distributed by ARM [1]. To run their kernels, users will execute:

  $ ./Foundation_v8 --image linux-system.axf --block-device raring-rootfs

To mount the raring-rootfs filesystem, the kernel parameter should
typically include:

  root=/dev/vda

For this device to be present, the kernel must be compiled with
VIRTIO_{MMIO,BLK}. To make this work out-of-the-box, make
VIRTIO_{MMIO,BLK} part of the default configuration.

[1]: http://www.arm.com/products/tools/models/fast-models/foundation-model.php