Documentation: bindings: add abracon,abx80x

Document the bindings for abracon,abx80x and related compatibles.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Philippe De Muyter <phdm@macqel.be>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/devicetree/bindings/rtc/abracon,abx80x.txt

+Abracon ABX80X I2C ultra low power RTC/Alarm chip
+
+The Abracon ABX80X family consist of the ab0801, ab0803, ab0804, ab0805, ab1801,
+ab1803, ab1804 and ab1805. The ab0805 is the superset of ab080x and the ab1805
+is the superset of ab180x.
+
+Required properties:
+
+ - "compatible": should one of:
+        "abracon,abx80x"
+        "abracon,ab0801"
+        "abracon,ab0803"
+        "abracon,ab0804"
+        "abracon,ab0805"
+        "abracon,ab1801"
+        "abracon,ab1803"
+        "abracon,ab1804"
+        "abracon,ab1805"
+	Using "abracon,abx80x" will enable chip autodetection.
+ - "reg": I2C bus address of the device
+
+Optional properties:
+
+The abx804 and abx805 have a trickle charger that is able to charge the
+connected battery or supercap. Both the following properties have to be defined
+and valid to enable charging:
+
+ - "abracon,tc-diode": should be "standard" (0.6V) or "schottky" (0.3V)
+ - "abracon,tc-resistor": should be <0>, <3>, <6> or <11>. 0 disables the output
+                          resistor, the other values are in ohm.