CONFIG_MACH_EXYNOS5_DT is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

SAMSUNG EXYNOS5 Machine using device tree

modulename: mach-exynos5-dt.ko

configname: CONFIG_MACH_EXYNOS5_DT

Linux Kernel Configuration
└─>SAMSUNG EXYNOS SoCs Support
└─>SAMSUNG EXYNOS5 Machine using device tree
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Machine support for Samsung EXYNOS5 machine with device tree enabled.
Select this if a fdt blob is available for the EXYNOS5 SoC based board.

source code:
arch/arm/mach-exynos/mach-exynos5-dt.c