watchdog: renesas_wdt: Add R-Car Gen4 support

Add the compatible string for the R-Car Gen4 family.
No further driver changes are needed.

Signed-off-by: Thanh Quan <thanh.quan.xn@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/cc395105e1d34aab2c076d368c0737833970b9d2.1642525158.git.geert+renesas@glider.be
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>