power: supply: bq27xxx: adds device tree binding documentation.

The bq27xxx binding is a standard i2c style binding, however the
deprecated compatible fields and different revisions warrant its own
documentation.
Signed-off-by: Chris Lapa <chris@lapa.com.au>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power/supply/bq27xxx.txt

+Binding for TI BQ27XXX fuel gauge family
+
+Required properties:
+- compatible: Should contain one of the following:
+ * "ti,bq27200" - BQ27200
+ * "ti,bq27210" - BQ27210
+ * "ti,bq27500" - deprecated, use revision specific property below
+ * "ti,bq27510" - deprecated, use revision specific property below
+ * "ti,bq27520" - deprecated, use revision specific property below
+ * "ti,bq27500-1" - BQ27500/1
+ * "ti,bq27510g1" - BQ27510-g1
+ * "ti,bq27510g2" - BQ27510-g2
+ * "ti,bq27510g3" - BQ27510-g3
+ * "ti,bq27520g1" - BQ27520-g1
+ * "ti,bq27520g2" - BQ27520-g2
+ * "ti,bq27520g3" - BQ27520-g3
+ * "ti,bq27520g4" - BQ27520-g4
+ * "ti,bq27530" - BQ27530
+ * "ti,bq27531" - BQ27531
+ * "ti,bq27541" - BQ27541
+ * "ti,bq27542" - BQ27542
+ * "ti,bq27546" - BQ27546
+ * "ti,bq27742" - BQ27742
+ * "ti,bq27545" - BQ27545
+ * "ti,bq27421" - BQ27421
+ * "ti,bq27425" - BQ27425
+ * "ti,bq27441" - BQ27441
+ * "ti,bq27621" - BQ27621
+- reg: integer, i2c address of the device.
+
+Example:
+
+bq27510g3 {
+    compatible = "ti,bq27510g3";
+    reg = <0x55>;
+};