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

IBM Legacy iSeries

configname: CONFIG_PPC_ISERIES

Linux Kernel Configuration
└─>IBM Legacy iSeries
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
selects
CONFIG_PPC_INDIRECT_IO

depends
CONFIG_PPC64
CONFIG_PPC_MULTIPLATFORM