CONFIG_ARCH_ORION5X_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Marvell Orion5x Flattened Device Tree

modulename: board-dt.ko

configname: CONFIG_ARCH_ORION5X_DT

Linux Kernel Configuration
└─>Marvell Orion
└─>Marvell Orion5x Flattened Device Tree
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Say 'Y' here if you want your kernel to support the
Marvell Orion5x using flattened device tree.

source code:
/arch/arm/mach-orion5x/board-dt.c