arm64: dts: renesas: r8a77980: add RWDT support

Describe RWDT in the R8A77980 SoC device tree.

Enable RWDT on the Condor and V3H Starter Kit boards.

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>