MAINTAINERS: Add Thermal subsystem co-maintainer

Adding myself as co-maintainer for the thermal subsystem.

The agreed work split will be so that as a co-maintainer
I will be taking care of platform thermal drivers, non-ACPI
thermal drivers and making sure the thermal framework core
code will continue to suffice the non-ACPI thermal needs.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>