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

IBM pSeries & new (POWER5-based) iSeries

modulename: udbg_16550.ko

configname: CONFIG_PPC_PSERIES

Linux/PowerPC Kernel Configuration
└─>Platform support
└─> IBM pSeries & new (POWER5-based) iSeries
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
arch/powerpc/kernel/udbg_16550.c