dt-bindings: rtc: Add property for Raspberry Pi 5 RTC

The Raspberry Pi 5 has a built-in RTC that can be battery-backed
via the VBAT pin. When a rechargeable coin cell is used, the
hardware requires a trickle-charge voltage to keep the battery
topped up.

Expose the trickle-voltage-millivolt DT property so the driver can
program the correct charge voltage for board specific configurations with
the attached battery.

The property is added to the existing raspberrypi,bcm2835-firmware
binding (rather than creating a new RTC node) because the RTC is
accessed exclusively through the firmware mailbox interface.
We only $ref the individual property from rtc.yaml and do not
reference the full schema, because the firmware node name does not
match the $nodename pattern required by rtc.yaml.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Sander Speetjens <sander.speetjens@gmail.com>
Link: https://patch.msgid.link/20261018151056.1051330-3-sander.speetjens@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/arm/bcm/raspberrypi,bcm2835-firmware.yaml

@@ -133,6 +133,11 @@ properties:
     required:
       - compatible
 
+  trickle-voltage-millivolt:
+    $ref: /schemas/rtc/rtc.yaml#/properties/trickle-voltage-millivolt
+    minimum: 1300
+    maximum: 5000
+
 required:
   - compatible
   - mboxes
@@ -144,6 +149,7 @@ examples:
     firmware {
         compatible = "raspberrypi,bcm2835-firmware", "simple-mfd";
         mboxes = <&mailbox>;
+        trickle-voltage-millivolt = <3300>;
 
         firmware_clocks: clocks {
             compatible = "raspberrypi,firmware-clocks";