PFK (Per-File-Key) driver architecture document.

PFK is a new driver, a part of eCryptfs enhanced by ICE solution
Platforms supported: MSM8994, MSM8996
Location: security/pfe

Change-Id: Id30c679b8b284fe31e12ce0820fb336c1608bc3a
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>