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 only v2 version.

Change-Id: Ifce120fcb349360e36c4ea74bcaf5ea14e1dece5
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>