CONFIG_ATARI_DSP56K is not available for the default architecture x86.
Result is shown for architecture m68k

Atari DSP56k support

modulename: dsp56k.ko
configname: CONFIG_ATARI_DSP56K

Linux Kernel Configuration
└─> Character devices
└─> Atari DSP56k support

If you want to be able to use the DSP56001 in Falcons, say Y. This
driver is still experimental, and if you don't know what it is, or
if you don't have this processor, just say N.

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

source code: