ARM: at91/dt: at91sam9n12: use slow clock where necessary

The watchdog, the reset controller, the RTC, the shutdown controller, the
timer counters and the LCD PWM need the slow clock, add it where necessary.
The LCD PWM will be handled later.

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