CONFIG_CPU_PXA300 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_CPU_PXA300 (not configurable)

modulename: pxa300.ko

configname: CONFIG_CPU_PXA300

Linux Kernel Configuration
└─>CPU_PXA300
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
PXA300 (codename Monahans-L)

source code: