msm: kgsl: Register thermal cooling device

Thermal framework has capability to mitigate any device registered with
the devfreq framework using a generic devfreq mitigation interface.
This mitigation interface will be used by the thermal framework to place
aggregated frequency cap and floor mitigation.

To allow thermal framework to mitigate GPU using devfreq framework,
register GPU as a devfreq based cooling device.

Change-Id: Ia1f954ccaac8de7eac5e383bed5b12e96c2669be
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>