ARM: shmobile: Add r8a7791 thermal device node to DTS

Hook up the r8a7791 thermal sensor to the DTS.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>