ARM: dts: msm: Mount system as root for msm8917

System needs to be mounted as root with the System-As-Root feature
for non-A/B builds. And for A/B the system is mounted from fstabs.

Change-Id: Ib78211e5ba70d8ff12c4a2f74b047017efeb08be
Signed-off-by: Monika Singh <monising@codeaurora.org>