arm64: dts: r8a7796: Add RWDT node

Add a device node for the Watchdog Timer (WDT) controller on the Renesas
R-Car M3-W (r8a7796) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>