default or selected kernelversion does not have config value CONFIG_LITEX_SUBREG_SIZE.
Result is shown for kernelversion 5.14.15

Size of a LiteX CSR subregister, in bytes


Linux Kernel Configuration
└─>Device Drivers
└─>SOC (System On Chip) specific Drivers
└─>Enable LiteX SoC Builder specific drivers
└─>Size of a LiteX CSR subregister, in bytes
In linux kernel since version 5.12 (release Date: 2021-04-25)  
LiteX MMIO registers (referred to as Configuration and Status
registers, or CSRs) are spread across adjacent 8- or 32-bit
subregisters, located at 32-bit aligned MMIO addresses. Use
this to select the appropriate size (1 or 4 bytes) matching
your particular LiteX build.