ion: store private heap data into ion pools

Store the private data of ion platform heap into pools so that
it can be used later by the pools to apply cache maintenance
on its pages by getting the right device.

Change-Id: I91fa99424d65802a69995ad2709f660faf3f5663
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>