ARM: dts: msm: Mount system and vendor for SDM450

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: Id4754402e15e0f2e63ca4f6b9340a738158bee71
Signed-off-by: Monika Singh <monising@codeaurora.org>