extcon: Add EXTCON_DISP_DP and the property for USB Type-C

Add EXTCON_DISP_DP for the Display external connector. For Type-C
connector the DisplayPort can work as an Alternate Mode(VESA DisplayPort
Alt Mode on USB Type-C Standard). The Type-C support both normal
and flipped orientation, so add a property to extcon.

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