power: supply: bq25890: Add bq25890_set_otg_cfg() helper

Add a bq25890_set_otg_cfg() helper function, this is a preparation
patch for adding regulator support.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>