build: mark DICE_PROTECTION_ENVIRONMENT as an experimental feature

The DICE_PROTECTION_ENVIRONMENT support has been marked as experimental
since it has not incorporated into the threat model.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ic1ea4d59fd8a3fde7ce0404488a56e9d11bc1b85
1 file changed