Add thermal support for R8A774B1 (RZ/G2N) SoC.

Based on the work done for r8a77965 SoC.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1569250648-33857-3-git-send-email-biju.das@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>