clk: renesas: r8a7794: Add rwdt clock

Add "rwdt" clock to r8a7794_mod_clks. Also, since we may need to access
the watchdog registers at any time, declare the clock as critical.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>