lp8727_charger: Support the device tree feature

The interrupt and charging parameters are configurable in the device tree
structure. In the board test, a GPIO is used for handling LP8727
interrupts. The device tree binding documentation is added also.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>