gpu: ion: Restrict access to CP heap

On certain targets, the CP heap should only be used
for secure allocations. Add a check to determine which
targets are allowed to make non-secure allocations from
the CP heap type. Targets with this restriction will
fall back to an alternate heap.

Change-Id: Ieaa9e76cbf2dc3ea1da6f4e75a4de903c39a3077
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>