rtc: bq32000: add trickle charger option, with device tree binding

BQ32000 devices have "trickle chargers".  Introduce a code to enable the
charger, based on device tree.

Without charger, RTC does not keep time after power off.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Pavel Machek <pavel@denx.de>
Cc: Jason Cooper <jason@lakedameon.net>
Cc: Matti Vaittinen <matti.vaittinen@nsn.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>