The fusb302 Type-C port-controller cannot control the current-limit
directly, so we need to exported the limit so that another driver
(e.g. the charger driver) can pick the limit up and configure the
system accordingly.

The power-supply subsys already provides infrastructure for this,
power-supply devices have the notion of being supplied by another
power-supply and have properties through which we can export the
current-limit.

Register a power_supply and export the current-limit through the
power_supply's current-max property.

Cc: "Yueyao (Nathan) Zhu" <yueyao@google.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>