drivers/rtc/rtc-isl12022.c: device tree support

Add support for configuring the ISL12022 real-time clock via the Device
tree framework.  This is based on what I've seen in the related ISL12057
driver, it has been tested and works on a Technologic Systems TS-7670
device which uses a ISL12020 RTC device, my device tree looks like this:

	apbx@80040000 {
		i2c0: i2c@80058000 {
			pinctrl-names = "default";
			pinctrl-0 = <&i2c0_pins_a>;
			clock-frequency = <400000>;
			status = "okay";

			isl12022@0x6f {
				compatible = "isl,isl12022";
				reg = <0x6f>;
			};
		};
		... etc
	};

Signed-off-by: Stuart Longland <stuartl@vrt.com.au>
Cc: Roman Fietze <roman.fietze@telemotive.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>