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

CPUfreq driver for i.MX CPUs

modulename: cpu_op-mx51.ko

and/or

cpufreq.ko

configname: CONFIG_CPU_FREQ_IMX

Linux Kernel Configuration
└─>CPU Power Management
└─>CPUfreq driver for i.MX CPUs
In linux kernel since version 2.6.21 (release Date: 2007-04-25)  
This enables the CPUfreq driver for i.MX CPUs.

source code:
arch/arm/mach-mx5/cpu_op-mx51.c
selects
CONFIG_CPU_FREQ_TABLE

depends
CONFIG_ARCH_HAS_CPUFREQ
CONFIG_ARCH_MXC
CONFIG_CPU_FREQ