ARM: dts: msm: Mount system and vendor for SDM439

Add avb flag when mounting system and vendor images
for non-multislot. When multislot is disabled system
is not the root folder hence early mounting to be
done using dtsi.

Change-Id: I8d523bfb634479ff56d6328559569f1c8d535e08
Signed-off-by: Monika Singh <monising@codeaurora.org>