CONFIG_MACH_SMDK6440 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_MACH_SMDK6440.
Result is shown for kernelversion 6.5.1

SMDK6440

modulename: mach-smdk6440.ko

configname: CONFIG_MACH_SMDK6440

Linux Kernel Configuration
└─>SMDK6440
In linux kernel since version 2.6.34 (release Date: 2010-05-16)  
Machine support for the Samsung SMDK6440

source code:
arch/arm/mach-s5p64x0/mach-smdk6440.c