Following the messages on the mailing list regarding the possible issue around
reading DTB's information, we decided to flag the fconf feature as experimental.
A uniform approach should be used to handle properties miss and DTB validation.

Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
Change-Id: Ib3c86e81fb2e89452c593f68d825d3d8f505e1fb