ARM: shmobile: Add r8a7791 thermal platform device

Add a thermal platform device for the legacy case
on the r8a7791 SoC. This keeps the r8a7791 in sync
with the r8a7790 sister device.

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>