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>
diff --git a/Documentation/devicetree/bindings/arm/atmel-at91.txt b/Documentation/devicetree/bindings/arm/atmel-at91.txt
index ad319f8..2e99b5b 100644
--- a/Documentation/devicetree/bindings/arm/atmel-at91.txt
+++ b/Documentation/devicetree/bindings/arm/atmel-at91.txt
@@ -46,10 +46,12 @@
   shared across all System Controller members.
 
 System Timer (ST) required properties:
-- compatible: Should be "atmel,at91rm9200-st"
+- compatible: Should be "atmel,at91rm9200-st", "syscon", "simple-mfd"
 - reg: Should contain registers location and length
 - interrupts: Should contain interrupt for the ST which is the IRQ line
   shared across all System Controller members.
+Its subnodes can be:
+- watchdog: compatible should be "atmel,at91rm9200-wdt"
 
 TC/TCLIB Timer required properties:
 - compatible: Should be "atmel,<chip>-tcb".