arm64: defconfig: Add Broadcom Vulcan to defconfig

Enable Broadcom Vulcan support in arm64 default configuration. This will
build the device tree needed to boot on a Broadcom Vulcan board.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>