ARM: dts: msm: Add avb flags for system and vendor

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: Id9754402e05e0f2e63ca0f6b9340a338158be331
Signed-off-by: Jiten Patel <jitepate@codeaurora.org>