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

Atari MFP serial support

configname: CONFIG_ATARI_MFPSER

Linux/68k Kernel Configuration
└─>Character devices
└─>Atari MFP serial support
In linux kernel since version 2.6.12  
If you like to use the MFP serial ports ("Modem1", "Serial1") under
Linux, say Y. The driver equally supports all kinds of MFP serial
ports and automatically detects whether Serial1 is available.

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

Note for Falcon users: You also have an MFP port, it's just not
wired to the outside... But you could use the port under Linux.

depends
CONFIG_ATARI