Documentation: dt-bindings: trickle charger dt binding document for ds1339

Some DS13XX devices have "trickle chargers".  Introduce a device tree
binding for the resistor and diode configuration for enabling trickle
charger.

Signed-off-by: Matti Vaittinen <matti.vaittinen@nsn.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pavel Machek <pavel@denx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>