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

PXA300 (codename Monahans-L)

modulename: zylonite_pxa300.ko

and/or

pxa300.ko

configname: CONFIG_CPU_PXA300

Linux Kernel Configuration
└─>Intel PXA2xx/PXA3xx Implementations
└─>Supported PXA3xx Processor Variants
└─>PXA300 (codename Monahans-L)
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
source code:
arch/arm/mach-pxa/pxa300.c
selects

CONFIG_PXA3xx