default or selected kernelversion does not have config value CONFIG_MTD_NAND_AT91_ECC_SOFT.
Result is shown for kernelversion 5.19-rc1

Software ECC

configname: CONFIG_MTD_NAND_AT91_ECC_SOFT

ECC management for NAND Flash / SmartMedia on AT91
└─>Software ECC
In linux kernel since version 2.6.26 (release Date: 2008-07-13)  
Uses software ECC.

NB : hardware and software ECC schemes are incompatible.
If you switch from one to another, you'll have to erase your
mtd partition.