extcon: Introduce EXTCON_PROP_USB_SS property for SuperSpeed mode

EXTCON_PROP_USB_SS (SuperSpeed)[1] is necessary to distinguish
between USB/USB2 and USB3 connections on USB Type-C cables.

[1] https://en.wikipedia.org/wiki/USB#Overview



Cc: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>