CONFIG_SOC_IMX31 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

i.MX31 support

modulename: clk-imx31.ko

and/or

mm-imx3.ko

configname: CONFIG_SOC_IMX31

Linux Kernel Configuration
└─>Freescale i.MX family
└─>i.MX31 support
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
This enables support for Freescale i.MX31 processor

source code:
/arch/arm/mach-imx/