ARM: dts: msm: Mount system and vendor for MSM8996

Add avb flag when mounting 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: Ib74874a07139f949c4592fd99b578c251da6cb5c
Signed-off-by: Monika Singh <monising@codeaurora.org>