clk: renesas: r8a7792: Add rwdt clock

Add "rwdt" clock to r8a7792_mod_clks. Also, since we may need to access
the watchdog registers at any time, declare the clock as critical.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>