ARM: at91/dt: sama5d3: 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.

[boris.brezillon@free-electrons.com: add tcb clocks]
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>