CONFIG_PPC_MPC831x is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

CONFIG_PPC_MPC831x (not configurable)

configname: CONFIG_PPC_MPC831x

Linux Kernel Configuration
└─>PPC_MPC831x
In linux kernel since version 2.6.21 (release Date: 2007-04-25)  

is selected by
CONFIG_GPIO_MPC8XXX

is depended by
CONFIG_MPC831x_RDB