MAINTAINERS: thermal: Change the git tree location

The thermal trees were merged into a single one shared with the
maintainer of the subsystem.

Update the location of this group git tree.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20191205121227.19203-1-daniel.lezcano@linaro.org