dt-bindings: rtc: add generic bindings

Generic device tree bindings for Real Time Clock devices

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This document describes generic bindings which can be used to describe Real Time

Clock devices in a device tree.

Required properties

-------------------

- compatible : name of RTC device following generic names recommended practice.

For other required properties e.g. to describe register sets,

clocks, etc. check the binding documentation of the specific driver.

Optional properties

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- start-year : if provided, the default hardware range supported by the RTC is

               shifted so the first usable year is the specified one.

The following properties may not be supported by all drivers. However, if a

driver wants to support one of the below features, it should adapt the bindings

below.

- trickle-resistor-ohms : Selected resistor for trickle charger. Should be given

                          if trickle charger should be enabled

- trickle-diode-disable : Do not use internal trickle charger diode Should be

                          given if internal trickle charger diode should be

                          disabled

- wakeup-source :         Enables wake up of host system on alarm