dt-bindings: watchdog: renesas-wdt: Add R-Car Gen2 support

This commit documents the compatibility with R-Car Gen2 and RZ/G
devices by defining the generic compatible string "renesas,rcar-gen2-wdt".
Also, this patch expands the list of SoC-specific compatible strings to
include RZ/G and R-Car Gen2 devices.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>