gpu: ion: Add support for carveout heaps on msm targets

Add infrastructure for supporting ion carveout heaps.
The memory type should be specified in the board file using
mach/ion.h. The ion platform driver will be responsible for
allocating the correct memory.

Change-Id: Id70ed1fd799f9a5a16ff971b18d0e868bc09d875
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>