ARM: shmobile: r8a7793: Add thermal device to DT

Instantiate the thermal sensor in the r8a7793 device tree.

Based on very similar work for the r8a7790 by Magnus Damm.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>