ARM: sunxi_defconfig: Enable CMA

commit c13e7f313da33d1488355440f1a10feb1897480a upstream.

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.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>