CONFIG_TILE_GXIO_UART is not available for the selected architecture x86.
Result is shown for architecture tile

CONFIG_TILE_GXIO_UART (not configurable)

modulename: uart.ko

configname: CONFIG_TILE_GXIO_UART

Linux Kernel Configuration
└─>TILE_GXIO_UART
In linux kernel since version 3.12 (release Date: 2013-11-03)
source code:
/arch/tile/gxio/

is depended by
CONFIG_SERIAL_TILEGX