dt-bindings: arm: atmel,at91rm9200-st: convert to DT schema

Convert System Timer binding to YAML format.
Changes during conversion:
- Add "#address-cells" and "#size-cells" to the properties and required as
  watchdog is defined as a child node to the timer parent node.
- Define watchdog as a pattern property along with unit address in
  examples.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Akhila YS <akhilayalmati@gmail.com>
Link: https://lore.kernel.org/r/20260227-arm-microchip-v4-4-7e2ae1c5b5d6@gmail.com
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>

Documentation/devicetree/bindings/arm/atmel,at91rm9200-st.yaml

@@ -0,0 +1,69 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/atmel,at91rm9200-st.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Atmel System Timer
+
+maintainers:
+  - Nicolas Ferre <nicolas.ferre@microchip.com>
+  - Claudiu Beznea <claudiu.beznea@tuxon.dev>
+
+description:
+  The System Timer (ST) module in AT91RM9200 provides periodic tick and
+  alarm capabilities. It is exposed as a simple multi-function device
+  (simple-mfd + syscon) because it shares its register space and interrupt
+  with other System Controller blocks.
+
+properties:
+  compatible:
+    items:
+      - const: atmel,at91rm9200-st
+      - const: syscon
+      - const: simple-mfd
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 1
+
+patternProperties:
+  "^watchdog@[0-9a-f]+$":
+    $ref: /schemas/watchdog/atmel,at91rm9200-wdt.yaml#
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/irq.h>
+    timer@fffffd00 {
+        compatible = "atmel,at91rm9200-st", "syscon", "simple-mfd";
+        reg = <0xfffffd00 0x100>;
+        interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;
+        clocks = <&slow_xtal>;
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        watchdog@fffffd40 {
+            compatible = "atmel,at91rm9200-wdt";
+            reg = <0xfffffd40 0x40>;
+        };
+    };
+...

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

@@ -1,14 +1,5 @@
 Atmel system registers
 
-System Timer (ST) required properties:
-- 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.
-- clocks: phandle to input clock.
-Its subnodes can be:
-- watchdog: compatible should be "atmel,at91rm9200-wdt"
-
 RAMC SDRAM/DDR Controller required properties:
 - compatible: Should be "atmel,at91rm9200-sdramc", "syscon" or
 			"atmel,at91sam9260-sdramc" or