arm64: defconfig: enable devtmpfs mount option

Matching x86 and making it more convenient to run the arm64 default
kernel as distros like Ubuntu need this option.

Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/configs/defconfig

@@ -56,6 +56,7 @@ CONFIG_NET_9P=y
 CONFIG_NET_9P_VIRTIO=y
 CONFIG_UEVENT_HELPER_PATH="/sbin/hotplug"
 CONFIG_DEVTMPFS=y
+CONFIG_DEVTMPFS_MOUNT=y
 CONFIG_DMA_CMA=y
 CONFIG_BLK_DEV_LOOP=y
 CONFIG_VIRTIO_BLK=y