ion: msm: Define heap IDs as constants in their own file

The ION heap IDs are also needed by the device-tree to
provide the kernel with the heap ID for the heap that
a device-tree entry describes. Currently, these IDs are
hard-coded, which can be error-prone when updating the
heap IDs.

Thus, create a header that can also be accessed by the
device-tree and userspace, and move the heap IDs to that
header. This makes it so that the device-tree can now use
the constants defined in the header, instead of hard-coded
IDs, making updates significantly easier.

Change-Id: I58b1aae9924504f2b20e159b9be8da7a2ee8d248
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>