thermal: rcar: Remove binding docs for r8a7794

The latest information that I have is that there is no thermal IP
block present on the r8a7794 SoC so remove the corresponding binding.

Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>