clk: renesas: r8a779f0: Add WDT clock

Add the module clock used by the RCLK Watchdog Timer (RWDT) on the
Renesas R-Car S4-8 (r8a779f0) SoC.  Mark it as a critical clock, to
ensure uninterrupted watchdog operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/8d9b280065a663f2cf31db7b21a010aa781a0af1.1642525158.git.geert+renesas@glider.be