power: supply: bq256xx: add kerneldoc for structure members

Document members of structure to fix W=1 warnings like:

  drivers/power/supply/bq256xx_charger.c:240: warning:
    Function parameter or member 'charger' not described in 'bq256xx_device'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>