ARM: at91/dt: declare atmel,at91rm9200-st as a syscon

The system timer register range is used to handle three different
functionalities: Periodic Interval Timer, Real Time Timer and watchdog. Declare
it as a syscon to be able to get a regmap.
Also declare it as a simple-mfd and its watchdog subnode.

Finally, document the watchdog compatible string.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>