ARM: shmobile: r8a7779 dtsi: Add SoC-specific thermal compatible property

The thermal node used the generic compatible property only.
Add the SoC-specific one, to make it future proof.

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