mysteries_intel: Add a section about SMM_BWP

Something to point users to when SMM_BWP might be causing problems.

Change-Id: I394c033e8d4ff96433162f86aefb428d8acf6349
Signed-off-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/36986


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>