ARM: dts: msm: Add force-enable-root-clk property to gpu_gx GDSC on msm8996

On msm8996, the graphics cores root clock, gfx3d_clk_src needs to
be force enabled prior to turning on the core. Once the GDSC is
enabled, the root clock can be force disabled. Add the
qcom,force-enable-root-clk property to allow doing so.

Change-Id: I56c2c1e227ba8f9602c845ab06fa9c5501b9befd
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>