ARM: dts: msm: add stub regulator devices for 8992

Add stub regulator devices for msm8992 because RPM regulator support
is not yet in place.

Add voltage corner stub-regulator devices for SMPS 1A (VDD_CX), SMPS 2A
(VDD_MX) and SMPS 2B (VDD_GFX) in order to support corner based consumers.

Change-Id: I8421cb0868aee3a53daba57181b3114bf6b7a68b
Signed-off-by: Ke Liu <keliu@codeaurora.org>