ARM: dts: overlays: Unify overlay CMA handling

Now that we don't have to abuse the kernel command line to change CMA's
size we can clean-up and centralize CMA usage in overlays.

A new file, cma-overlay.dts is created to be used as a standalone
overlay or included on other overlays. All CMA users are converted to
this scheme. Ultimately upstream-overlay.dts is also updated to use the
default CMA size provided by upstream.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>