dma-buf: heaps: cma: Turn the heap into a module

dma-buf: heaps: cma: Turn the heap into a module

All the symbols used by the CMA heap are already exported, so turning it
into a module is straightforward. We only need to add the usual MODULE_*
macros, import the proper namespaces and change the Kconfig symbol to a
tristate.

This heap won't be able to unload though, since we're missing a lot of
infrastructure to make it safe.

Reviewed-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Andrew Davis <afd@ti.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patch.msgid.link/20260427-dma-buf-heaps-as-modules-v5-2-b6f5678feefc@kernel.org