ARM: dts: msm: Enlarge default CMA region to 20M

There is a request to support secure device which needs to load
another DP image. So additional 4M space required.
Make the change to enlarge default CMA region to 20M.

Change-Id: I02e62514c69eea33266cb8eee33cd828ee4eba8d
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>