drivers: thermal: parent virtual hwmon with thermal zone

When  creating virtual hwmon devices based out of thermal
zone devices, the virtual devices won't have parents.

This patch changes the code so that the parent of virtual
hwmon devices is the thermal zone device that they are
based of.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>