flashy-0536abf

spec: add flash layout for Bletchley and Greatlakes

Summary:
We didn't specify the requirements on the flash layout
for Bletchley and Greatlakes.  Add it now.

Test Plan: Doc only.

Reviewed By: garnermic

fbshipit-source-id: 3b8254cf2ac475b7f5b07c965fda278787b4c94e