ARM: dts: msm: Add alias name for secondary GPU sensor in MSM8998

Add alias name for the secondary GPU temperature sensor in MSM8998. This
alias information will be used by thermal-engine.

Change-Id: I4d3400efef8886153018435468526f88418ee0a0
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>