dma: contiguous: Reserve default CMA heap

dma: contiguous: Reserve default CMA heap

The CMA code, in addition to the reserved-memory regions in the device
tree, will also register a default CMA region if the device tree doesn't
provide any, with its size and position coming from either the kernel
command-line or configuration.

Let's register that one for use to create a heap for it.

Reviewed-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://lore.kernel.org/r/20251013-dma-buf-ecc-heap-v8-4-04ce150ea3d9@kernel.org