CONFIG_ARCH_U300 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

ST-Ericsson U300 Series

modulename: clk-u300.ko

configname: CONFIG_ARCH_U300

ARM system type
└─>ST-Ericsson U300 Series
In linux kernel since version 2.6.31 (release Date: 2009-09-09)  
Support for ST-Ericsson U300 series mobile platforms.

source code:
drivers/clk/clk-u300.c

depends
CONFIG_MMU or CONFIG_ARCH_MULTI_V5 or &

is selected by
CONFIG_MACH_U300