CONFIG_SOC_IMX21 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_SOC_IMX21 (not configurable)

modulename: mm-imx21.ko

and/or

clk-imx21.ko

configname: CONFIG_SOC_IMX21

Linux Kernel Configuration
└─>Freescale i.MX support
└─>SOC_IMX21
In linux kernel since version 2.6.36 (release Date: 2010-10-20)  
source code:
/arch/arm/mach-imx/

depends
CONFIG_ARCH_MXC

is selected by
CONFIG_PINCTRL_IMX21

is depended by
CONFIG_MACH_MX21ADS