charger: Adds CHARGER_PROP_INPUT_REGULATION_CURRENT_UA prop

Adds support for setting the target threshold for an input current
limit regulated by an analog loop. The analog loop responds by
throttling the charge current limit as necessary.

These are distinct from comparator based overcurrent protection
thresholds that isolated the charger from the power source when
triggered.

Signed-off-by: Ricardo Rivera-Matos <ricardo.rivera-matos@cirrus.com>