clk: renesas: r8a77980: Add RCLK for watchdog timer

On R-Car V3H, RCLK can be switched between EXTALR and the On-Chip
Oscillator using mode pin MD19.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>