thermal: rcar_gen3_thermal: Register hwmon sysfs interface

Register the hwmon sysfs interface on R-Car Gen3 thermal driver to
align it with Gen2 driver. Use devm_add_action() to unregister the
hwmon interface automatically.

Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: linux-renesas-soc@vger.kernel.org
To: linux-pm@vger.kernel.org
From: Marek Vasut <marek.vasut+renesas@gmail.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>