ARM: firmware: enable Trusted Foundations by default

As discussed previously (https://lkml.org/lkml/2013/11/26/289

), enable
Trusted Foundation support by default since it already depends on a
supporting architecture being selected.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Stephen Warren <swarren@nvidia.com>