CONFIG_MIPS_EXTERNAL_TIMER is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_MIPS_EXTERNAL_TIMER (not configurable)

configname: CONFIG_MIPS_EXTERNAL_TIMER

Linux Kernel Configuration
└─>MIPS_EXTERNAL_TIMER
In linux kernel since version 2.6.33 (release Date: 2010-02-24)  
is depended by
CONFIG_CS5536_MFGPT