default or selected kernelversion does not have config value CONFIG_SERIAL_IFX6X60.
Result is shown for kernelversion 6.9.9

SPI protocol driver for Infineon 6x60 modem (EXPERIMENTAL)

modulename: ifx6x60.ko

configname: CONFIG_SERIAL_IFX6X60

Linux Kernel Configuration
└─>Device Drivers
└─>Character devices
└─>Serial drivers
└─>SPI protocol driver for Infineon 6x60 modem (EXPERIMENTAL)
In linux kernel since version 2.6.38 (release Date: 2011-03-14)  
Support for the IFX6x60 modem devices on Intel MID platforms.

source code: