arm64: defconfig: Enable R-Car thermal driver

Enable the R-Car thermal driver as a built-in.
This seems safer than enabling as a module from the point of view of
protecting equipment from overheating.

This driver is used in conjunction with the R-Car V3M (r8a77970),
E3 (r8a77990) and D3 (r8a77995) SoCs.

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