gpu: ion: Add support for device tree.

Add device tree parsing to common msm ion code
to allow new targets to specify ION heap configuration
in device tree.

Document the device tree bindings for ION.

Change-Id: I40b7f4b1fbb3ca4ad1d6dc06a33b647f6af1d669
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
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>