Revert "staging: android: ion: Rework heap registration"

This reverts commit 2f87f50b ("staging: android: ion: Rework heap
registration/enumeration").

Revert ion heap registration rework so that we can keep our ION DT
support.

Change-Id: Ic0087cd845767a899cc4475de693995fcc67c8b6
Signed-off-by: Liam Mark <lmark@codeaurora.org>