Add thermal support for R8A774A1 (RZ/G2M) SoC.

Based on the work done for r8a7796 SoC.
Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>