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

ST SPEAr Family

configname: CONFIG_PLAT_SPEAR

Linux Kernel Configuration
└─>ST SPEAr Family
In linux kernel since version 2.6.35 (release Date: 2010-08-01)  

is selected by
CONFIG_GPIO_SPEAR_SPICS