ARM: at91/dt: at91sam9261: use slow clock where necessary

The watchdog, the reset controller, the real-time timer, the shutdown
controller and the timer counter need the slow clock, add it where
necessary.

Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>