thermal: of: use for_each_available_child_of_node for child iterator

Use for_each_available_child_of_node() for iterating over each
available child instead of iterating over each child and then
checking their status.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>