add private field in ion_heap and ion_platform_heap structure

copy private field from platform configuration to internal heap structure.

Change-Id: Ia7571d88fc2f72f5d655fb6f6b54fde389d96c85
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
[laura: Rebase context fixes]
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>