ARM: dts: msm: add dt support for etm save and restore for 8939

Add device tree entries for ETM save-restore across power collapse
since these ETM registers do not retain their states across power collapse.

Change-Id: I6345fa513eff84f86181979f3eb5f0c90e31c8b9
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>