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

CONFIG_PPC_MULTIPLATFORM (not configurable)

modulename: prom_init.ko

configname: CONFIG_PPC_MULTIPLATFORM

Linux Kernel Configuration
└─>Platform support
└─>PPC_MULTIPLATFORM
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
arch/powerpc/kernel/prom_init.c
depends
CONFIG_6xx or CONFIG_PPC64

is selected by
CONFIG_PPC_86xx