clk: renesas: r8a7796: Add watchdog core clocks

Add all core clocks related to the Watchdog Timer (WDT) controller on
the Renesas R-Car M3-W (r8a7796) SoC: OSC, Internal RCLK, and RCLK.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>