bq20z75: Add support for external notification

Adding support for external power change notification. One problem found
is that there is a lag time before the sensor will return a new status.
To ensure that we only fire off the power_supply_changed event when the
status returned from the sensor is actually different, we delay sending
the the notification, and instead poll on it looking for a change. The
amount of time to poll is configurable via platform data.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>