X-Power AXP288 PMIC Fuel Gauge Driver

New power_supply driver at driver/power which interfaces with the
axp20x mfd driver as a cell. Provides battery info, monitors for
changes, and generates alerts on temperature and capacity issues

Signed-off-by: Todd Brandt <todd.e.brandt@linux.intel.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>