ARM: dts: msm: Add dummy regulator support for msm8992

Add a dummy regulator device on msm8992, since some devices
may require dummy regulators for rails that are always
powered on.

Change-Id: If79cdaacae78c16a1ffdf270d98b03841ff4c4c3
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>