ARM: dts: msm: Mount system as root for msm8937

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: I9c1e889cb7a7864aac6c51375b742324bac5d6ba
Signed-off-by: Monika Singh <monising@codeaurora.org>