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Intersil ISL12057 I2C RTC/Alarm chip

ISL12057 is a trivial I2C device (it has simple device tree bindings,
consisting of a compatible field, an address and possibly an interrupt
line).

Nonetheless, it also supports an option boolean property
("wakeup-source") to handle the specific use-case found
on at least three in-tree users of the chip (NETGEAR ReadyNAS 102, 104
and 2120 ARM-based NAS); On those devices, the IRQ#2 pin of the chip
(associated with the alarm supported by the driver) is not connected
to the SoC but to a PMIC. It allows the device to be powered up when
RTC alarm rings. In order to mark the device has a wakeup source and
get access to the 'wakealarm' sysfs entry, this specific property can
be set when the IRQ#2 pin of the chip is not connected to the SoC but
can wake up the device.

Required properties supported by the device:

 - "compatible": must be "isil,isl12057"
 - "reg": I2C bus address of the device

Optional properties:

 - "wakeup-source": mark the chip as a wakeup source, independently of
    the availability of an IRQ line connected to the SoC.
    (Legacy property supported: "isil,irq2-can-wakeup-machine")

 - "interrupt-parent", "interrupts": for passing the interrupt line
   of the SoC connected to IRQ#2 of the RTC chip.


Example isl12057 node without IRQ#2 pin connected (no alarm support):

	isl12057: isl12057@68 {
		compatible = "isil,isl12057";
		reg = <0x68>;
	};


Example isl12057 node with IRQ#2 pin connected to main SoC via MPP6 (note
that the pinctrl-related properties below are given for completeness and
may not be required or may be different depending on your system or
SoC, and the main function of the MPP used as IRQ line, i.e.
"interrupt-parent" and "interrupts" are usually sufficient):

		    pinctrl {
				...

				rtc_alarm_pin: rtc_alarm_pin {
					marvell,pins = "mpp6";
					marvell,function = "gpio";
				};

				...

		    };

	...

	isl12057: isl12057@68 {
		compatible = "isil,isl12057";
		reg = <0x68>;
		pinctrl-0 = <&rtc_alarm_pin>;
		pinctrl-names = "default";
		interrupt-parent = <&gpio0>;
		interrupts = <6 IRQ_TYPE_EDGE_FALLING>;
	};


Example isl12057 node without IRQ#2 pin connected to the SoC but to a
PMIC, allowing the device to be started based on configured alarm:

	isl12057: isl12057@68 {
		compatible = "isil,isl12057";
		reg = <0x68>;
		wakeup-source;
	};