CONFIG_PPC_OF is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_PPC_OF (not configurable)

modulename: of_device.ko

and/or

of_platform.ko

configname: CONFIG_PPC_OF

Linux/PowerPC Kernel Configuration
└─>PPC_OF
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
arch/powerpc/kernel/
is selected by
CONFIG_SERIAL_PMACZILOG