CONFIG_HAVE_AT91_DBGU2 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_HAVE_AT91_DBGU2 (not configurable)

configname: CONFIG_HAVE_AT91_DBGU2

Linux Kernel Configuration
└─>HAVE_AT91_DBGU2
In linux kernel since version 3.18 (release Date: 2014-12-07)  

is depended by
CONFIG_SOC_SAMA5D4