arm64: dts: meson-gx: increase default shared CMA pool size

Devices using the new V4L2 mem2mem vdec require a larger CMA pool. As
nearly all GX* devices are video/media focused and will use it, set a
larger (256MB) default value.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>