charger: Adds input current notifier properties

Some charger devices (such as BQ25710 and ISL9241) issue
notifications to the system via a dedicated line (such as
the PROCHOT signal on x86 platforms) to almost instantly
throttle the system load to prevent a brownout event.

This patch introduces two properties to configure two
distinct input current notification events of different
severity.

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