doc: Set fconf as experimental feature

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

docs/about/features.rst

@@ -73,6 +73,8 @@ Current features
    to be configured at runtime if required by the platform. It also enables
    loading of a hardware configuration (for example, a kernel device tree)
    as part of the FIP, to be passed through the firmware stages.
+   This feature is now incorporated inside the firmware configuration framework
+   (fconf), which is still flagged as experimental.
 
 -  Support for alternative boot flows, for example to support platforms where
    the EL3 Runtime Software is loaded using other firmware or a separate

docs/getting_started/build-options.rst

@@ -643,6 +643,7 @@ Common build options
 -  ``ARM_IO_IN_DTB``: This flag determines whether to use IO based on the
    firmware configuration framework. This will move the io_policies into a
    configuration device tree, instead of static structure in the code base.
+   This is currently an experimental feature.
 
 
 -  ``USE_ROMLIB``: This flag determines whether library at ROM will be used.