ARM: dts: msm: specify VDD_CX corners for each VDD_GFX corner on msm8996

Due to a hardware dependency on MSM8996 versions below 3.1,
VDD_CX must be kept at or above the SVS corner whenever VDD_GFX
operates at the Turbo corner.  Specify pm8994_s1_corner (VDD_CX)
as the system-supply for the VDD_GFX cpr3-regulator device.  Also
define the appropriate VDD_CX corner to use for each VDD_GFX
corner.

Change-Id: I5c7d8673e2477cf803639c3f1277960e09df6bf7
Signed-off-by: David Collins <collinsd@codeaurora.org>