ion: carveout: Add secure carveout heap type

A secure carveout heap manages memory that may never be accessed by HLOS.
Several pools of differing types of secure memory may be selected from
based on the flags argument to ion_alloc().

Change-Id: Ieb3d043ab7d5a0e88a32042f9ba24b3535a863ec
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>