msm: kgsl: Show max gpu temperature

Parse the thermal zone names from gpu DT to get each zone's temperature
and print the maximum temperature among them in sysfs.

CRs-Fixed: 2765368
Change-id: I37767d1bf9a8d4887dfcbad56ebdf17d27e4e465
Signed-off-by: Young Hwan Kwak <ykwak@codeaurora.org>