power: supply: axp20x_battery: add support for AXP717

Add support for the AXP717 PMIC battery charger. The AXP717 differs
greatly from existing AXP battery chargers in that it cannot measure
the discharge current. The datasheet does not document the current
value's offset or scale, so the POWER_SUPPLY_PROP_CURRENT_NOW is left
unscaled.

Tested-by: Philippe Simons <[email protected]>
Signed-off-by: Chris Morgan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sebastian Reichel <[email protected]>

Author: macromorgan