ARM: dts: msm: Add MSMCOBALT v2 specific gfx_stub regulator changes

Use the gfx_stub_vreg regulator node for sourcing the graphics
clocks and GDSC on the pre-silicon platforms. Note that CPR
changes are still required for the graphics rail on the actual
target.

CRs-Fixed: 1015446
Change-Id: Ib92b11c23785e737f91ad09bcbb3d0b849d3a2bf
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>