staging: typec: tcpm: Add get_current_limit tcpc_dev callback

A Rp signalling the default current limit indicates that we're possibly
connected to an USB2 power-source. In some cases the type-c port-controller
may provide the capability to detect the current-limit in this case,
through e.g. BC1.2 detection.

This commit adds an optional get_current_limit tcpc_dev callback which
allows the port-controller to provide current-limit detection for when
the CC pin is pulled up with Rp.

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>