ARM: sunxi_defconfig: Enable CMA

The DRM driver most notably, but also out of tree drivers (for now) like
the VPU or GPU drivers, are quite big consumers of large, contiguous memory
buffers. However, the sunxi_defconfig doesn't enable CMA in order to
mitigate that, which makes them almost unusable.

Enable it to make sure it somewhat works.

Cc: <stable@vger.kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>