staging: ion: msm: Only parse available dt nodes

ION is currently ignoring the status property of device tree
heap nodes. Make ION aware of the status of the ION heap nodes making
it possible to disable certain heaps.

Change-Id: I96e6af97df6bbd1c24ace9aa476baae28409ea96
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>