Documentation: dt: atmel-at91: add clocks to system timer, rstc and shdwc

The system timer (at91rm9200), the reset controller and the shutdown
controller need an input clock. This is the slow clock and they will not
function without it.

Also fix the shutdown controller example.

Acked-By: Sebastian Reichel <sre@kernel.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>