regulator/mfd: max8998: Document charger regulator

max8998 has a current regulator for control of its charging
current.  Document it.

Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Link: https://lore.kernel.org/r/BN6PR04MB06600D05B27BAE69970E3C30A3BA0@BN6PR04MB0660.namprd04.prod.outlook.com


Signed-off-by: Mark Brown <broonie@kernel.org>