CONFIG_ETRAX_SYNCHRONOUS_SERIAL is not available for thedefaultarchitecture x86.
Result is shown for architecture cris
default or selected kernelversion does not have config value CONFIG_ETRAX_SYNCHRONOUS_SERIAL.
Result is shown for kernelversion 6.7-rc3

Synchronous serial-port support

modulename: sync_serial.ko


Linux Kernel Configuration
└─>Drivers for built-in interfaces
└─>Synchronous serial-port support
In linux kernel since version 2.6.13 (release Date: 2005-08-28)  
Select this to enable the synchronous serial port driver.

source code: