soc: renesas: rcar-rst: Enable watchdog as reset trigger for Gen2

This patch allows for platform specific quirks as some of the SoC need
further customization for the watchdog to work properly, like for R-Car
Gen2 and for RZ/G.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>