ARM: dts: msm: Add CX regulator Phandle to MDP node

The Phandle might change for different versions of the msm8974 chip.
Add the Phandle of CX regulator to msm8974-pro version to prevent
vddMin in static image.

Change-Id: I0c740e8866a78b01cca4f7dc836a2d0c190cf28b
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>
Signed-off-by: Vinu Deokaran <vinud@codeaurora.org>