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

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>