bq20z75: Add optional battery detect gpio

Adding support for an optional gpio for battery detection. This is
passed in through the i2c platform data. It also accepts another
field, battery_detect_present to signify the gpio state which means
the battery is present, either 0 (low) or 1 (high).

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