ARM: dts: msm: add vref_rgmii regulator device for pmxpoorwills

Add a regulator device for vref_rgmii peripheral so that
clients such as emac can vote for it.

Change-Id: I4e7c140d8882bcecbc0f6ed1d241159432dea07b
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>