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

Support RealView(R) Platform Baseboard Explore

modulename: realview_pbx.ko

configname: CONFIG_MACH_REALVIEW_PBX

Linux Kernel Configuration
└─>RealView platform type
└─>Support RealView(R) Platform Baseboard Explore
In linux kernel since version 2.6.31 (release Date: 2009-09-09)  
Include support for the ARM(R) RealView(R) Platform Baseboard
Explore.

source code: