ARM: dts: msm: Add vreg property in npu power level nodes

Add vreg property in npu power level nodes to indicate the
actual power level for each set of clock frequency settings.

Change-Id: I07ba2893c09ef97d4b5d280350fdbec1428e9ead
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>