thermal: rcar: add rcar_zone_to_priv() macro

This patch adds rcar_zone_to_priv()
which is a helper macro for gettign private data.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>