CONFIG_ATARI_MIDI is not available for thedefaultarchitecture x86.
Result is shown for architecture m68k

Atari MIDI serial support

configname: CONFIG_ATARI_MIDI

Linux/68k Kernel Configuration
└─>Character devices
└─>Atari MIDI serial support
In linux kernel since version 2.6.12  
If you want to use your Atari's MIDI port in Linux, say Y.

To compile this driver as a module, choose M here.

depends
CONFIG_ATARI