ARM: dts: msm: Add the missing gpubw device for MSM8996

The gpubw device is needed to vote for the GPU bus bandwidth.
Without this the vbif governor will not be called.

Change-Id: Ie53718f9a9edec00e49a25d6794caa81e425fd18
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>