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>

Documentation/devicetree/bindings/arm/atmel-at91.txt

@@ -46,10 +46,12 @@ PIT Timer required properties:
   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".

arch/arm/boot/dts/at91rm9200.dtsi

@@ -356,9 +356,13 @@
 			};
 
 			st: timer@fffffd00 {
-				compatible = "atmel,at91rm9200-st";
+				compatible = "atmel,at91rm9200-st", "syscon", "simple-mfd";
 				reg = <0xfffffd00 0x100>;
 				interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;
+
+				watchdog {
+					compatible = "atmel,at91rm9200-wdt";
+				};
 			};
 
 			rtc: rtc@fffffe00 {