default or selected kernelversion does not have config value CONFIG_MTD_MAINSTONE.
Result is shown for kernelversion 5.15.113

CFI Flash device mapped on Intel Mainstone XScale eval board

modulename: mainstone-flash.ko

configname: CONFIG_MTD_MAINSTONE

Linux Kernel Configuration
└─>Device Drivers
└─>Memory Technology Devices (MTD)
└─>Mapping drivers for chip access
└─>CFI Flash device mapped on Intel Mainstone XScale eval board
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This provides a driver for the on-board flash of the Intel
'Mainstone PXA27x evaluation board.

source code:
drivers/mtd/maps//mainstone-flash.c