rtc: ds1307: add trickle charger device tree binding

rtc: ds1307: add trickle charger device tree binding

Some DS13XX devices have "trickle chargers".  Introduce a device tree
binding for specifying the trickle charger configuration for ds1339.

Only ds1339 dt binding is supported because this is the only chip I have.
I _assume_ the code would have worked on other allready supported chips.
However I cannot check the resistor values for the other chips or test
them.  For other chips the driver code works as earlier Eg.  it does not
check the dt bindings at all

Signed-off-by: Matti Vaittinen <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Pawel Moll <[email protected]>
Cc: Jason Cooper <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Pavel Machek <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>