ARM: dts: msm: add PM8916 stub regulators for MSM8939

Stub regulators have no dependencies on hardware, add stub regulator
devices so that regulator consumers can reference them.

Change-Id: I10eca8325e280178da9fc7d2753bbd005567918f
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>