* Raspberry Pi RTC

This is a Linux interface to an RTC managed by firmware, hence it's
virtual from a Linux perspective.

The interface uses the firmware mailbox api to access the RTC registers.

Required properties:
compatible: should be "raspberrypi,rpi-rtc"
firmware:   Reference to the RPi firmware device node.

Optional property:
trickle-charge-microvolt: specify a trickle charge voltage for the backup
                          battery in microvolts.

Example:

	rpi_rtc: rpi_rtc {
		compatible = "raspberrypi,rpi-rtc";
		firmware = <&firmware>;
		trickle-charge-microvolt = <3000000>;
	};