dt-bindings: rtc: add reset-source property

Some RTCs, e.g. the pcf2127, can be used as a hardware watchdog. But
if the reset pin is not actually wired up, the driver exposes a
watchdog device that doesn't actually work.

Provide a standard binding that can be used to indicate that a given
RTC can perform a reset of the machine, similar to wakeup-source.
Suggested-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201218101054.25416-2-rasmus.villemoes@prevas.dk

Documentation/devicetree/bindings/rtc/rtc.yaml

@@ -63,6 +63,11 @@ properties:
     description:
       Enables wake up of host system on alarm.
 
+  reset-source:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description:
+      The RTC is able to reset the machine.
+
 additionalProperties: true
 
 ...