doc: dma-buf: List the heaps by name

doc: dma-buf: List the heaps by name

Since we're going to introduce multiple instances of the CMA heap
driver, there's no single CMA heap anymore.

Let's use the heap name instead to differentiate between all the heaps
available in the system.

While we're at it, let's also rework the backward compatibility part to
make it easier to amend later on.

Reviewed-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
  [sumits: rebased to latest Doc]
Link: https://lore.kernel.org/r/20251013-dma-buf-ecc-heap-v8-1-04ce150ea3d9@kernel.org