ARM: shmobile: r8a7790: add thermal driver support

The current temperature may be read using:
cat /sys/class/thermal/thermal_zone0/temp

Based on similar work for the r8a73a4 by Kuninori Morimoto.

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