ARM: dts: Mount system as root for msm8909

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: Icdb62273726bf5b435844e4a540ae4a3ff4cc83b
Signed-off-by: Monika Singh <monising@codeaurora.org>