CONFIG_MACH_SAMA5_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Atmel SAMA5 Evaluation Kits with device-tree support

modulename: board-dt-sama5.ko

configname: CONFIG_MACH_SAMA5_DT

Linux Kernel Configuration
└─>Atmel AT91 System-on-Chip
└─>Atmel SAMA5 Evaluation Kits with device-tree support
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Select this if you want to experiment device-tree with
an Atmel Evaluation Kit.

source code: