This happens because CONFIG_NONPROMISC_DEVMEM (pre
CONFIG_STRICT_DEVMEM or CONFIG_X86_PAT are set in your kernel.
Although flashrom can do nothing about any of these, I will add this
information to the error message, so I'm not closing the ticket just yet.
I did not try it but this kernel patch may fix the issue with CONFIG_X86_PAT
since flashrom tries to mmap the first MB in cbtable.c: