MAINTAINERS: add thermal bindings entry in thermal domain

After discussion and agreement of thermal device tree bindings,
it is desirable to properly maintain thermal bindings for
existing and upcoming devices.

As original author of thermal device tree bindings, I am
volunteering to maintain them. This then adds and entry
for device tree bindings under thermal domain.

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>