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

CONFIG_PLAT_PXA (not configurable)

configname: CONFIG_PLAT_PXA

Linux Kernel Configuration
└─>System Type
└─>PLAT_PXA
In linux kernel since version 2.6.30 (release Date: 2009-06-09)  
is selected by
CONFIG_SND_PXA_SOC_SSP