ARM: dts: msm: Mount system and vendor for 8917

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 from device tree.

Change-Id: I5a08740060b03cbf7e730a171152ef79997fec9e
Signed-off-by: Naitik Bharadiya <bharad@codeaurora.org>