thermal: rcar: enable to use thermal-zone on DT

This patch enables to use thermal-zone on DT if it was calles as
"renesas,rcar-thermal-gen2".
Previous style (= non thermal-zone) is still supported by
"renesas,rcar-thermal" to keep compatibility for "git bisect".

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>