ARM: dts: sunxi: Change default CMA pool node name

The CMA node has a unit address, but no reg property which generates a
warning in DTC. Change the node name to reflect its usage and drop the unit
address.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>