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

CONFIG_PLAT_IOP (not configurable)

configname: CONFIG_PLAT_IOP

Linux Kernel Configuration
└─>System Type
└─>PLAT_IOP
In linux kernel since version 2.6.19 (release Date: 2006-11-29)  
selects
CONFIG_GENERIC_CLOCKEVENTS

is selected by
CONFIG_IOP_WATCHDOG