gpu: ion: Add support for specifying heap type in the device tree

Currently, there is no way to swap out the underlying heap type for a
given heap ID without modifying msm_ion.c and recompiling the
kernel. This is inconsistent with the rest of Ion which is customized
through the device tree. This change eliminates the need to modify and
recompile the kernel for heap type changes (although the boot image
will still need to be regenerated).

Add a device tree binding to specify the heap_type for Ion heaps.

Change-Id: I0a2b48299c829a828764d74a1d8e3c8f97de706d
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>