arm64: disable Broadcom Vulcan platform

Disable the option ARCH_VULCAN as a step towards deleting it entirely.
There is still a reference in the kernel tree for ARCH_VULCAN, so we
have to keep the config option around until that reference is removed.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Olof Johansson <olof@lixom.net>