CONFIG_ARCH_VEXPRESS_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Device Tree support for Versatile Express platforms

configname: CONFIG_ARCH_VEXPRESS_DT

Linux Kernel Configuration
└─>Versatile Express platform type
└─>Device Tree support for Versatile Express platforms
In linux kernel since version 3.4 (release Date: 2012-05-20)  
New Versatile Express platforms require Flattened Device Tree to
be passed to the kernel.

This option enables support for systems using Cortex processor based
ARM core and logic (FPGA) tiles on the Versatile Express motherboard,
for example:

- CoreTile Express A5x2 (V2P-CA5s)
- CoreTile Express A9x4 (V2P-CA9)
- CoreTile Express A15x2 (V2P-CA15)
- LogicTile Express 13MG (V2F-2XV6) with A5, A7, A9 or A15 SMMs
(Soft Macrocell Models)
- Versatile Express RTSMs (Models)

You must boot using a Flattened Device Tree in order to use these
platforms. The traditional (ATAGs) boot method is not usable on
these boards with this option.

If your bootloader supports Flattened Device Tree based booting,
say Y here.