dt/flattree: Add stub defintions for flat device tree function

Several flattened device tree functions are currently accessed in a
file that is compiled regardless of whether device tree is enabled
or not. Add stubs for these functions if device tree is not selected.

Change-Id: I0bea3325692fac46d3e5ef457aa30e083f71cd6d
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>